This manual (2 March 2015) is for Libmarpa 8.3.0.
Copyright © 2015 Jeffrey Kegler.
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
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This is essentially a reference document, but its early chapters lay out concepts essential to the others. Readers will usually want to read the chapters up and including Introduction to the external interface in order. Otherwise, they should follow their interests.
This document is very far from self-contained. It assumes the following:
Marpa::R2
or Marpa::R3
,
in Perl.
This document assumes some acquaintance with parsing theory. The reader's level of knowledge is probably adequate if he can answer the following questions, either immediately or after a little reflection.
Libmarpa implements the Marpa parsing algorithm. Marpa is named after the legendary 11th century Tibetan translator, Marpa Lotsawa. In creating Marpa, I depended heavily on previous work by Jay Earley, Joop Leo, John Aycock and Nigel Horspool.
Libmarpa implements the entire Marpa algorithm. This library does the necessary grammar preprocessing, recognizes the input, and produces parse trees. It also supports the ordering, iteration and evaluation of the parse trees.
Libmarpa is very low-level. For example, it has no strings. Rules, symbols, and token values are all represented by integers. This, of course, will not suffice for many applications. Users will very often want names for the symbols, non-integer values for tokens, or both. Typically, applications will use arrays to translate Libmarpa's integer ID's to strings or other values as required.
Libmarpa also does not implement most of the semantics. Libmarpa does have an evaluator (called a “valuator”), but it does not manipulate the stack directly. Instead, Libmarpa, based on its traversal of the parse tree, passes optimized step by step stack manipulation instructions to the upper layer. These instructions indicate the token or rule involved, and the proper location for the true token value or the result of the rule evaluation. For rule evaluations, the instructions include the stack location of the arguments.
Marpa requires most semantics to be implemented in the application. This allows the application total flexibility. It also puts the application is in a much better position to prevent errors, to catch errors at runtime or, failing all else, to successfully debug the logic.
The classes of Libmarpa's object system fall into two types: major and numbered. These are the Libmarpa's major classes, in sequence.
The major objects have one letter abbreviations, which are used frequently. These are, in the standard sequence,
All of Libmarpa's major classes, except the configuration class, are “time” classes. Except for objects in the grammar class, all time objects are created from another time object. Each time object is created from a time object of the class before it in the sequence. A recognizer cannot be created without a precomputed grammar; a bocage cannot be created without a recognizer; and so on.
When one time object is used to create a second time object, the first time object is the parent object and the second time object is the child object. For example, when a bocage is created from a recognizer, the recognizer is the parent object, and the bocage is the child object.
Grammars have no parent object. Every other time object has exactly one parent object. Value objects have no child objects. All other time objects can have any number of children, from zero up to a number determined by memory or some other machine-determined limit.
Every time object has a base grammar. A grammar object is its own base grammar. The base grammar of a recognizer is the grammar that it was created with. The base grammar of any other time object is the base grammar of its parent object. For example, the base grammar of a bocage is the base grammar of the recognizer that it was created with.
Every object in a “time” class has its own, distinct, lifetime, which is controlled by the object's reference count. Reference counting follows the usual practice. Contexts which take a share of the “ownership” of an object increase the reference count by 1. When a context relinquishes its share of the ownership of an object, it decreases the reference count by 1.
Each class of time object has a “ref” and an “unref”
method, to be used by those contexts which need to
explicitly increment and decrement the reference count.
For example, the “ref” method for the grammar class is
marpa_g_ref()
and the “unref” method for the grammar class is
marpa_g_unref()
.
Time objects do not have explicit destructors. When the reference count of a time object reaches 0, that time object is destroyed.
Much of the necessary reference counting is performed automatically. The context calling the constructor of a time object does not need to explicitly increase the reference count, because Libmarpa time objects are always created with a reference count of 1.
Child objects “own” their parents, and when a child object is successfully created, the reference count of its parent object is automatically incremented to reflect this. When a child object is destroyed, it automatically decrements the reference count of its parent.
In a typical application, a calling context needs only to remember to “unref” each time object that it creates, once it is finished with that time object. All other reference decrements and increments are taken care of automatically. The typical application never needs to explicitly call one of the “ref” methods.
More complex applications may find it convenient to have one or more contexts share ownership of objects created in another context. These more complex situations are the only cases in which the “ref” methods will be needed.
In addition to its major, “time” objects, Libmarpa also has numbered objects. Numbered objects do not have lifetimes of their own. Every numbered object belongs to a time object, and is destroyed with it. Rules and symbols are numbered objects. Tokens values are another class of numbered objects.
In traditional Earley parsers, the concept of location is very simple. Locations are numbered from 0 to n, where n is the length of the input. Every location has an Earley set, and vice versa. Location 0 is the start location. Every location after the start location has exactly one input token associated with it.
Some applications do not fit this traditional input model — natural language processing requires ambiguous tokens, for example. Libmarpa allows a wide variety of alternative input models.
This document assumes that the reader knows the concepts
behind Libmarpa's
alternative input models, either from the documentation
of a higher level interface, such as
Marpa::XS
,
Marpa::R2
,
or Marpa::R3
,
or from Marpa's
theory document.
As a reminder, in Libmarpa a location is called a earleme. The number of an Earley set is the ID of the Earley set, or its ordinal. In the traditional model, the ordinal of an Earley set and its earleme are always exactly the same, but in Libmarpa they will be different.
The important earleme variables are the current earleme, the furthest earleme
and the latest earleme.
The current earleme is the earleme that Libmarpa is currently working on.
More specifically, it is the one at which new tokens will start.
Since tokens are never zero length, a new token will always end after the
current earleme.
The current earleme is initially earleme 0.
Every call to marpa_r_earleme_complete()
advances the
current earleme by 1.
The furthest earleme is the highest numbered (and therefore “furthest”)
earleme at which a token ends.
The furthest earleme is initially earleme 0.
With every call to marpa_r_alternative()
, the end of the token
it adds is calculated.
A token ends at the earleme location current+length,
where current is the current earleme,
and length is the length of the newly added token.
After a call to marpa_r_alternative()
,
the furthest earleme is its value before the call,
or current+length,
whichever is greater.
The latest earleme is the earleme of the latest Earley set. The latest Earley set is the last Earley set completed. This is always the highest numbered Earley set. If there is an Earley set at the current earleme, it is the latest Earley set and the latest earleme is equal to the current earleme. There is never an Earley set after the current earleme.
After every call to the marpa_r_earleme_complete()
method
that adds a token,
the value of the latest earleme is
same as the value of the current earleme.
After every call to the marpa_r_earleme_complete()
method
that does not add a token,
the value of the latest earleme is unchanged
from its value before the call.
The current earleme tracks the advance of the recognizer through the input.
Input tokens always start at the current earleme.
An application can advance past the current earleme,
by calling marpa_r_earleme_complete()
, which
increments the current earleme by 1.
After initialization,
marpa_r_earleme_complete()
is
the only way to manipulate the value of the current earleme.
The furthest earleme tracks how “far out” tokens can be found.
In the standard input model, calling
marpa_r_earleme_complete()
after each
marpa_r_alternative()
call is sufficient to process
all inputs,
and the furthest earleme's value
can be typically be ignored.
In alternative input models, if tokens have lengths greater than
1, calling
marpa_r_earleme_complete()
once after the last token
is read may not be enough to ensure that all tokens have been processed.
To ensure that all tokens have been processed,
an application must advance the current earleme
by calling marpa_r_earleme_complete()
,
until the current earleme is equal to the furthest earleme.
The lastest earleme is the earleme of the last Earley set. The latest earleme is different from the current earleme if and only if there is no Earley set at the current earleme. A different end of parsing can be specified, but by default, parsing is of the input in the range from earleme 0 to the latest earleme.
All input models have the same initial values. Initially the current, latest and furthest earleme are always earleme 0.
Understanding the settings of current, latest and furthest earleme is crucial to working with advanced input models, and for this reason the next sections will go through the possibilities carefully. The presentation will start with the most traditional and restrictive models. It will proceed to less restrictive models.
In the standard model of input,
calls to marpa_r_alternative()
and marpa_r_earleme_complete()
are
made in pairs.
There is, first, exactly one call
to marpa_r_alternative()
,
and it is for a token with length 1.
Following it must be a call
to marpa_r_earleme_complete()
.
For an input of length n, there will be
exactly n such paired calls.
Suppose, in the standard model that,
for a call
to marpa_r_alternative()
,
the following is true:
marpa_r_alternative()
,
the following will be true:
Suppose, in the standard model that,
for a call
to marpa_r_earleme_complete()
,
the following is true:
marpa_r_earleme_complete()
,
the current, latest and furthest earlemes will
be the same.
Specifically,
the following will be true:
As a first loosening of the standard model,
we no longer require calls to marpa_r_alternative()
to be paired with calls to
marpa_r_earleme_complete()
.
Instead,
we allow a series of one or more calls
to marpa_r_alternative()
before each call to
marpa_r_earleme_complete()
.
We still require that there be at least one call
to marpa_r_alternative()
before each call to
marpa_r_earleme_complete()
,
and we still require that all tokens have
a length of 1.
In the ambiguous input model, the behavior of the current,
latest and furthest earlemes are exactly
as described for the standard model, with one minor
exception:
Where the current earleme is c,
and a call to marpa_r_alternative()
is not the first
of a series,
the value of the furthest earleme before the call will be
c+1.
Our next loosening of the restrictions is to allow variable length tokens. That is, instead of requiring that all tokens be of length 1, we allow tokens to be of length 1 or longer. This does change the behavior of the earleme variables.
Suppose, in the variable length token model that,
for a call
to marpa_r_alternative()
,
the following is true:
marpa_r_alternative()
,
the following will be true:
marpa_r_alternative()
.
Suppose, in the variable length token model that,
for a call
to marpa_r_earleme_complete()
,
the following is true:
marpa_r_earleme_complete()
,
the following will be true:
marpa_r_earleme_complete()
.
To fully generalize the input model,
we now need to remove only one restriction.
We now allow empty earlemes — earlemes with
no tokens and no Earley set.
For the generalized input model,
the effect on the earleme variables of
a call
to marpa_r_alternative()
is exactly
the same as it is for the variable length input model.
The effect on the earleme variables of a call to
to marpa_r_earleme_complete()
depends on
whether or not that call creates an empty earleme.
A call
to marpa_r_earleme_complete()
creates an empty earleme if and only if
it falls into one of these two cases:
marpa_r_alternative()
since
recognizer initialization.
marpa_r_alternative()
since
the previous call
to marpa_r_earleme_complete()
.
Suppose, in the generalized input model that,
for a call
to marpa_r_earleme_complete()
that creates an empty earleme,
the following is true:
marpa_r_earleme_complete()
,
the following will be true:
marpa_r_earleme_complete()
.
Suppose, in the generalized input model that,
for a call
to marpa_r_earleme_complete()
that does not creates an empty earleme,
the following is true:
marpa_r_earleme_complete()
,
the following will be true:
marpa_r_earleme_complete()
.
At this point, the most generalized input model has been introduced. Here are some useful facts about the earleme variables that will always be the case, no matter which of the input models is in use.
If the parser is not exhausted, the following will always be true, no matter which of the input models is in use.
In an exhausted parser, the following will always be true, no matter which of the input models is in use.
A terminal symbol is a symbol which may appear in the input. Traditionally, all LHS symbols, as well as the start symbol, must be non-terminals. This is Marpa's behavior, by default.
Marpa allows the user to eliminate the distinction between terminals and non-terminals. In this, it differs from traditional parsers. Libmarpa can arrange for a terminal to appear on the LHS of one or more rules, or for a terminal to be the start symbol. However, since terminals can never be zero length, it is a logical contradiction for a nulling symbol to also be a terminal and Marpa does not allow it.
Token values are int
's.
Libmarpa does nothing with token values except accept
them from the application and return them during
parse evaluation.
Libmarpa handling of semantics is unusual. Most semantics are left up to the application, but Libmarpa guides them. Specifically, the application is expected to maintain the evaluation stack. Libmarpa's valuator provides instructions on how to handle the stack. Libmarpa's stack handling instructions are called “steps”. For example, a Libmarpa step might tell the application that the value of a token needs to go into a certain stack position. Or a Libmarpa step might tell the application that a rule is to be evaluated. For rule evalution, Libmarpa will tell the application where the operands are to be found, and where the result must go.
The detailed discussion of Libmarpa's handling of semantics is in the reference chapters of this document, under the appropriate methods and classes. The most extensive discussion of the semantics is in the section that deals with the methods of the value time class. See Value methods.
Libmarpa is thread-safe, given circumstances as described below. The Libmarpa methods are not reentrant.
Libmarpa is C89-compliant. It uses no global data, and calls only the routines that are defined in the C89 standard and that can be made thread-safe. In most modern implementations, the default C89 implementation is thread-safe to the extent possible. But the C89 standard does not require thread-safety, and even most modern environments allow the user to turn thread safety off. To be thread-safe, Libmarpa must be compiled and linked in an environment that provides thread-safety.
While Libmarpa can be used safely across multiple threads, a Libmarpa grammar cannot be. Further, a Libmarpa time object can only be used safely in the same thread as its base grammar. This is because all time objects with the same base grammar share data from that base grammar.
To work around this limitation, the same grammar definition can be used to a create a new Libmarpa grammar time object in each thread. If there is sufficient interest, future versions of Libmarpa could allow thread-safe cloning of grammars and other time objects.
Libmarpa avoids fatal errors,
leaving that decision up to the application,
with one exception.
Currently, if malloc
fails to allocate memory,
Libmarpa terminates the program with a fatal error.
While this is in keeping with current practice, future versions of Libmarpa are likely to both allow an alternative memory allocator to be specified, and to allow the user to specifier a handler to be called when an out-of-memory condition occurs.
The following chapters describe Libmarpa's external interface in detail.
The reference sections for each major class begin with an overview. These sections name the most important Libmarpa methods in the order in which they are typically used, and very briefly describe their purpose. The overviews are intended as “cheat sheets”.
The overview sections often speak of an “archetypal” application. The archetypal Libmarpa application implements a complete logic flow, starting with the creation of a grammar, and proceeding all the way to the return of the final result from a value object. In the archetypal Libmarpa application, the grammar, input and semantics are all small but non-trivial.
Return values are discussed method by method, but some general practices are worth mentioning. For methods that return an integer, Libmarpa usually reserves −1 for special purposes, such as indicating loop termination in an iterator. In Libmarpa, methods usually indicate failure by returning −2. Any result greater than −2 usually indicates success.
If a method returns an pointer value,
NULL
usually indicates failure.
Any other result usually indicates success.
On failure, a Libmarpa method always sets the error code. On success, a Libmarpa method may take one of three actions:
MARPA_ERROR_NONE
.
Which of the three actions the method takes on success should be regarded as unspecified, unless stated in the description of the method in this document. If a method sets an informational error code on success, that will always be stated in its description, and the description will specify, for each informational error code, the specific error code value, the circumstances under which it is set, and what return values will accompany it.
Informational error codes are set because some applications may find them convenient. Typically, informational error codes are redundant information, which may be ignored.
A method can have many reasons for failing, and many of the reasons for failure are common to large number of methods. Full descriptions of the error codes that are returned by the external methods are given in their own section. See External error codes.
Methods in Libmarpa follow a strict naming convention.
All methods have a name beginning with marpa_
,
if they are part of the
external interface.
If an external method is not a static method,
its name is prefixed with one of
marpa_c_
,
marpa_g_
,
marpa_r_
,
marpa_b_
,
marpa_o_
,
marpa_t_
or
marpa_v_
,
where the single letter between underscores
is one of the Libmarpa major class abbreviations.
The letter indicates which class
the method belongs to.
Methods that are exported,
but that are part of
the internal interface,
begin with _marpa_
.
Methods that are part of the internal interface
(often called “internal methods”)
are subject to change and are intended for use
only by Libmarpa's developers.
Libmarpa reserves the marpa_
and _marpa_
prefixes for itself,
with all their capitalization variants.
All Libmarpa names visible outside the package
will begin with a capitalization variant
of one of these two prefixes.
Checks that the Marpa library in use is compatible with the given version. Generally you will pass in the constants
MARPA_MAJOR_VERSION
,MARPA_MINOR_VERSION
,MARPA_MICRO_VERSION
as the three arguments to this function; that produces a check that the library in use is compatible with the version of Libmarpa the application or module was compiled against. Compatibility is defined by two things: first the version of the running library is newer than the version required_major.required_minor.required_micro. Second the running library must be binary compatible with the version required_major.required_minor.required_micro (same major version.)Return value:
MARPA_ERR_NONE
if the Marpa library is compatible with the requested version. If the library is not compatible, one of the following is returned, indicating the nature of the mismatch:
MARPA_ERR_MAJOR_VERSION_MISMATCH
,MARPA_ERR_MINOR_VERSION_MISMATCH
MARPA_ERR_MICRO_VERSION_MISMATCH
Returns the version number in version, which must have room for three
int
's.Return value: A non-negative number on success, −2 on failure.
The configuration object is intended for future extensions.
These may
allow the application to override Libmarpa's memory allocation
and fatal error handling without resorting to global
variables, and therefore in a thread-safe way.
Currently, the only function of the Marpa_Config
class is to give marpa_g_new()
a place to put its error code.
Marpa_Config
is Libmarpa's only “major”
class which is not a time class.
There is no constructor or destructor, although
Marpa_Config
objects do need to be initialized
before use.
Aside from its own accessor,
Marpa_Config
objects are only used by marpa_g_new
and no reference to their location is not kept
in any of Libmarpa's time objects.
The intent is to that it be convenient
to have them in memory that might be deallocated
soon after marpa_g_new
returns.
For example, they could be put on the stack.
Initialize the config information to “safe” default values. Unspecified behavior will result if an initialized configuration is used to create a grammar.
Return value: A non-negative value. Always succeeds.
Error codes are usually kept in the base grammar, which leaves
marpa_g_new()
no place to put its error code on failure. Objects of theMarpa_Config
class provide such a place. p_error_string is reserved for use by the internals. Applications should set it toNULL
.Return value: The error code in config. Always succeeds.
An archetypal application has a grammar.
To create a grammar, use the marpa_g_new()
method.
When a grammar is no longer in use, its memory can be freed
using the
marpa_g_unref()
method.
To be precomputed,
a grammar must have one or more symbols.
To create symbols, use the
marpa_g_symbol_new()
method.
To be precomputed,
a grammar must have one or more rules.
To create rules, use the
marpa_g_rule_new()
and
marpa_g_sequence_new()
methods.
For non-trivial parsing,
one or more of the symbols must be terminals.
To mark a symbol as a terminal,
use the
marpa_g_symbol_is_terminal_set()
method.
To be precomputed,
a grammar must have exactly one start symbol.
To mark a symbol as the start symbol,
use the
marpa_g_start_symbol_set()
method.
Before parsing with a grammar, it must be precomputed.
To precompute a grammar,
use the
marpa_g_precompute()
method.
Creates a new grammar time object. The returned grammar object is not yet precomputed, and will have no symbols and rules. Its reference count will be 1.
Unless the application calls
marpa_c_error
, Libmarpa will not reference the location pointed to by the configuration argument aftermarpa_g_new
returns. The configuration argument may beNULL
, but if it is, there will be no way to determine the error code on failure.Return value: On success, the grammar object. On failure,
NULL
, and the error code is set in configuration.
It is recommended that this call be made immediately after the grammar constructor. It turns off a deprecated feature.
The
marpa_g_force_valued
forces all the symbols in a grammar to be “valued”. The opposite of a valued symbol is one about whose value you do not care. This distinction has been made in the past in hope of gaining efficiencies at evaluation time. Current thinking is that the gains do not repay the extra complexity.Return value: On success, a non-negative integer. On failure, a negative integer.
Increases the reference count by 1. Not needed by most applications.
Return value: On success, the grammar object it was called with;
NULL
on failure.
Decreases the reference count by 1, destroying g once the reference count reaches zero.
Returns current value of the start symbol of grammar g. The value is that specified in the
marpa_g_start_symbol_set()
call, if there has been one.Return value: On failure, −2; −1 if there is no start symbol yet; otherwise the ID of the new start symbol.
Sets the start symbol of grammar g to symbol id.
Return value: On success, the ID of the new start symbol. If sym_id is well-formed, but there is no such symbol, −1. On failure, −2.
Return value: On success, the numerically largest symbol ID of g. On failure, −2.
A symbol is accessible if it can be reached from the start symbol.
Return value: On success, 1 if symbol sym_id is accessible, 0 if not. If sym_id is well-formed, but there is no such symbol, −1. If the grammar is not precomputed, or on other failure, −2.
A symbol is nullable if it sometimes produces the empty string. A nulling symbol is always a nullable symbol, but not all nullable symbols are nulling symbols.
Return value: On success, 1 if symbol sym_id is nullable, 0 if not. If sym_id is well-formed, but there is no such symbol, −1. If the grammar is not precomputed, or on other failure, −2.
A symbol is nulling if it always produces the empty string.
Return value: On success, 1 if symbol sym_id is nulling, 0 if not. If sym_id is well-formed, but there is no such symbol, −1. If the grammar is not precomputed, or on other failure, −2.
A symbol is productive if it can produce a string of terminals. All nullable symbols are considered productive.
Return value: On success, 1 if symbol sym_id is productive, 0 if not. If sym_id is well-formed, but there is no such symbol, −1. If the grammar is not precomputed, or on other failure, −2.
This return value of this call indicates whether sym_id is the start symbol.
Success value: Returns 1 if sym_id is the start symbol. Returns 0 if sym_id is not the start symbol, either because the start symbol is different from sym_id, or because the start symbol has not been set yet.
Failure values: If sym_id is well-formed, but there is no such symbol, −1. On other failure, −2.
These methods, respectively, set and query the “terminal status” of a symbol. To be used as an input symbol in the
marpa_r_alternative()
method, a symbol must be a terminal. This function flags symbol sym_id as a terminal if value is 1, or flags it as a non-terminal if value is 0.Once set to a value with the
marpa_g_symbol_is_terminal_set()
method, the terminal status of a symbol is “locked” at that value. A subsequent call tomarpa_g_symbol_is_terminal_set()
that attempts to change the terminal status of sym_id to a value different from its current one will fail. The error code will beMARPA_ERR_TERMINAL_IS_LOCKED
.By default, a symbol is a terminal if and only if it does not appear on the LHS of any rule. An attempt to flag a nulling symbol as a terminal will cause a failure, but this is not necessarily detected before precomputation.
Success returns: When successful, these methods return 1 if symbol sym_id is a terminal symbol after the call, 0 otherwise.
Failure returns: If sym_id is well-formed, but there is no such symbol, −1. On all other failures, −2. Other failures include when value is not 0 or 1; when the terminal status is locked and value is different from its current value; and, in the case of
marpa_g_symbol_is_terminal_set()
, when the grammar g is precomputed.
Creates a new symbol. The symbol ID will be a non-negative integer.
Return value: On success, the ID of a new symbol; On failure, −2.
Return value: On success, the numerically largest rule ID of g. On failure, −2.
A rule is accessible if it can be reached from the start symbol. A rule is accessible if and only if its LHS symbol is accessible. The start rule is always an accessible rule.
Return value: On success, 1 if rule rule_id is accessible, 0 if not. If rule_id is well-formed, but there is no such rule, −1. If the grammar is not precomputed, or on other failure, −2.
A rule is nullable if it sometimes produces the empty string. A nulling rule is always a nullable rule, but not all nullable rules are nulling rules.
Return value: On success, 1 if rule ruleid is nullable, 0 if not. If rule_id is well-formed, but there is no such rule, −1. If the grammar is not precomputed, or on other failure, −2.
A rule is nulling if it always produces the empty string.
Return value: On success, 1 if rule ruleid is nulling, 0 if not. If rule_id is well-formed, but there is no such rule, −1. If the grammar is not precomputed, or on other failure, −2.
A rule is a loop rule if it non-trivially produces the string of length one which consists only of its LHS symbol. Such a derivation takes the parse back to where it started, hence the term “loop”. “Non-trivially” means the zero-step derivation does not count — the derivation must have at least one step.
The presence of a loop rule makes a grammar infinitely ambiguous, and applications will typically want to treat them as fatal errors. But nothing forces an application to do this, and Marpa will successfully parse and evaluate grammars with loop rules.
Return value: On success, 1 if rule rule_id is a loop rule, 0 if not. If rule_id is well-formed, but there is no such rule, −1. If the grammar is not precomputed, or on other failure, −2.
A rule is productive if it can produce a string of terminals. An rule is productive if and only if all the symbols on its RHS are productive. The empty string counts as a string of terminals, so that a nullable rule is always a productive rule. For that same reason, an empty rule is considered productive.
Return value: On success, 1 if rule rule_id is productive, 0 if not. If rule_id is well-formed, but there is no such rule, −1. If the grammar is not precomputed, or on other failure, −2.
The length of a rule is the number of symbols on its RHS.
Return value: On success, the length of rule rule_id. On failure, −2.
Return value: On success, the LHS symbol of rule rule_id. If rule_id is well-formed, but there is no such rule, −1. On other failure, −2.
Creates a new external BNF rule in grammar g. The ID of the new rule will be a non-negative integer, which will be unique to that rule. In addition to BNF rules, Marpa also allows sequence rules, which are created by another method:
marpa_g_sequence_new()
. Sequence rules and BNF rules are numbered in the same series, so that a BNF rule will never have the same rule ID as a sequence rule, and vice versa.The LHS symbol is lhs_id, and there are length symbols on the RHS. The RHS symbols are in an array pointed to by rhs_ids.
Possible failures, with their error codes, include:
MARPA_ERR_SEQUENCE_LHS_NOT_UNIQUE
: The LHS symbol is the same as that of a sequence rule.MARPA_ERR_DUPLICATE_RULE
: The new rule would duplicate another BNF rule. Another BNF rule is considered the duplicate of the new one, if its LHS symbol is the same as symbol lhs_id, if its length is the same as length, and if its RHS symbols match one for one those in the array of symbols rhs_ids.Return value: On success, the ID of new external rule. On failure, −2.
Returns the ID of the symbol in position ix in the RHS of rule rule_id. The RHS position, ix, is zero-based.
Return value: On success, the ID of the symbol in position ix on the rules RHS. If rule_id is well-formed, but there is no such rule, −1. If ix is greater than or equal to the length of the rule, or on other failure, −2.
Success returns:
marpa_g_rule_is_proper_separation()
succeeds if and only if rule_id is valid. If rule rule_id is a sequence rule where the proper separation flag is set, returns 1. On other success, including the case where rule rule_id is not a sequence rule, returns 0.The
marpa_g_rule_is_proper_separation()
does not distinguish rules without proper separation from rules for which separation is not defined, because they are not sequence rules. Applications for which this distinction is important should use themarpa_g_sequence_min()
method.Failure returns: If rule_id is well-formed, but there is no such rule, returns −1. On other failure, −2.
Returns the mininum length of a sequence rule. This accessor can be also be used to test whether or not a rule is a sequence rule. −1 is returned if and only if the rule is valid but not a sequence rule.
Return value: If rule rule_id is a sequence rule, its minimum length. If rule rule_id is valid, but is not the rule ID of sequence rule, −1. If rule_id is well-formed, but there is no such rule, or on other failure, −2.
Adds a new sequence rule to grammar g. The ID of the new sequence rule will be a non-negative integer, which is unique to that rule. Sequence rules and BNF rules are numbered in the same series, so that a BNF rule will never have the same rule ID as a sequence rule, and vice versa.
The sequence is lhs_id, and the item to be repeated in the sequence is rhs_id. The sequence must be repeated at least min times, where min is 0 or 1. If separator_id is non-negative, it is a separator symbol.
If
flags & MARPA_PROPER_SEPARATION
is non-zero, separation is “proper”, that is, a trailing separator is not allowed. The term proper is based on the idea that properly-speaking, separators should actually separate items.Some higher-level Marpa interfaces offer the ability to discard separators in the semantics, and in fact will do this by default. At the Libmarpa level, sequences always “keep separators”. It is up to the programmer to arrange to discard separators, if that is what is desired.
The sequence RHS, or item, is restricted to a single symbol, and that symbol cannot be nullable. If separator_id is a symbol, it also cannot be a nullable symbol. Nullables on the RHS of sequences are restricted because they lead to highly ambiguous grammars. Grammars of this kind are allowed by Libmarpa, but they must be expressed using BNF rules, not sequence rules. This is for two reasons: First, sequence optimizations would not work in the presence of nullables. Second, since it is not completely clear what an application intends when it asks for a sequence of identical items, some of which are nullable, the user's intent can be more clearly expressed directly in BNF.
The LHS symbol cannot be the LHS of any other rule, whether a BNF rule or a sequence rule. On an attempt to create an sequence rule with a duplicate LHS,
marpa_g_sequence_new()
fails, setting the error code toMARPA_ERR_SEQUENCE_LHS_NOT_UNIQUE
.Sequence rules do not add to the classes of grammar parsed by Libmarpa — a sequence can always be written as BNF rules. When a rule is created with the
marpa_g_sequence_new()
method, Libmarpa will understand that it is a sequence, and will optimize accordingly. The speedup is often considerable.Return value: On success, the ID of the external rule. On failure, −2.
Return value: If rule rule_id is a sequence rule, its separator. If rule rule_id is a sequence rule, but there is no separator, −1. If rule_id is not a sequence rule, does not exist or is not well-formed; or on other failure, −2.
A symbol is counted if it appears on the RHS of a sequence rule, or if it is used as the separator symbol of a sequence rule.
Success return: Returns 1 if symbol sym_id is counted, 0 if not.
Failure return: If sym_id is well-formed, but there is no such symbol, −1. If sym_id is not well-formed, and on other failure, −2.
These methods, respectively, set and query the rank of a rule rule_id. When rule_id is created, its rank initialized to the default rank of the grammar.
Initially, the default rank of the grammar is 0. Methods to reset the grammar's default rank are currently in the experimental stage.
Return value: On success, returns the rank after the call, and sets the error code to
MARPA_ERR_NONE
. On failure, returns −2, and sets the error code to an appropriate value, which will never beMARPA_ERR_NONE
. Note that when the rank is −2, the error code is the only way to distinguish success from failure. The error code can be determined by using themarpa_g_error()
call.
These methods, respectively, set and query the “null ranks high” setting of the rule rule_id. The “null ranks high” setting is either 0 or 1. When rule_id is created, its “null ranks high” setting is initialized to 0.
The “null ranks high” setting affects the ranking of rules with properly nullable symbols on their right hand side. If a rule has properly nullable symbols on its RHS, each instance in which it appears in a parse will have a pattern of nulled and non-nulled symbols. Such a pattern is called a “null variant”.
If the “null ranks high” setting is 0 (the default), nulled symbols rank low. If the “null ranks high” setting is 1, nulled symbols rank high. Ranking of a null variants is done from left-to-right.
The
marpa_g_rule_null_high_set()
method will return failure after the grammar has been precomputed. If there is no other cause of failure, themarpa_g_rule_null_high()
method succeeds on both precomputed and unprecomputed grammars.Success return value: The value of the “null ranks high” flag after the call.
Failure return value: If rule_id is well-formed, but there is no such rule, −1. On all other failures, −2.
Allows the user to deactivate and reactivate symbol completion events in the grammar. When a recognizer is created, the activation status of each of its events is initialized to the activation status of that event in the base grammar. If reactivate is zero, the event is deactivated in the grammar. If reactivate is one, the event is activated in the grammar.
Symbol completion events are active by default if the symbol was set up for completion events in the grammar. If a symbol was not set up for completion events in the grammar, symbol completion events are inactive by default and any attempt to change that is a fatal error.
The activation status of a completion event in the grammar can only be changed if the symbol is marked as a completion event symbol in the grammar, and before the grammar is precomputed. However, if a symbol is marked as a completion event symbol in the recognizer, the completion event can be deactivated and reactivated in the recognizer.
Success cases: On success, the method returns the value of reactivate. The method succeeds trivially if the symbol is already set as indicated by reactivate.
Failure cases: If the active status of the completion event for sym_id cannot be set as indicated by reactivate, the method fails. On failure, −2 is returned.
Allows the user to deactivate and reactivate symbol nulled events in the grammar. When a recognizer is created, the activation status of each of its events is initialized to the activation status of that event in the base grammar. If reactivate is zero, the event is deactivated in the grammar. If reactivate is one, the event is activated in the grammar.
Symbol nulled events are active by default if the symbol was set up for nulled events in the grammar. If a symbol was not set up for nulled events in the grammar, symbol nulled events are inactive by default and any attempt to change that is a fatal error.
The activation status of a nulled event in the grammar can only be changed if the symbol is marked as a nulled event symbol in the grammar, and before the grammar is precomputed. However, if a symbol is marked as a nulled event symbol in the recognizer, the nulled event can be deactivated and reactivated in the recognizer.
Success cases: On success, the method returns the value of reactivate. The method succeeds trivially if the symbol is already set as indicated by reactivate.
Failure cases: If the active status of the nulled event for sym_id cannot be set as indicated by reactivate, the method fails. On failure, −2 is returned.
Allows the user to deactivate and reactivate symbol prediction events in the grammar. When a recognizer is created, the activation status of each of its events is initialized to the activation status of that event in the base grammar. If reactivate is zero, the event is deactivated in the grammar. If reactivate is one, the event is activated in the grammar.
Symbol prediction events are active by default if the symbol was set up for prediction events in the grammar. If a symbol was not set up for prediction events in the grammar, symbol prediction events are inactive by default and any attempt to change that is a fatal error.
The activation status of a prediction event in the grammar can only be changed if the symbol is marked as a prediction event symbol in the grammar, and before the grammar is precomputed. However, if a symbol is marked as a prediction event symbol in the recognizer, the prediction event can be deactivated and reactivated in the recognizer.
Success cases: On success, the method returns the value of reactivate. The method succeeds trivially if the symbol is already set as indicated by reactivate.
Failure cases: If the active status of the prediction event for sym_id cannot be set as indicated by reactivate, the method fails. On failure, −2 is returned.
Libmarpa can be set up to generate an
MARPA_EVENT_SYMBOL_COMPLETED
event whenever the symbol is completed. A symbol is said to be completed when a non-nulling rule with that symbol on its LHS is completed.For completion events to occur, the symbol must be marked as a completion event symbol. The
marpa_g_symbol_is_completion_event_set()
function marks symbol sym_id as a completion event symbol if value is 1, and unmarks it it as a completion event symbol if value is 0. Themarpa_g_symbol_is_completion_event()
method returns the current value of the completion event marking for symbol sym_id.Marking a completion event sets its activation status to on. Unmarking a completion event sets its activation status to off. The completion event marking cannot be changed once the grammar is precomputed.
If a completion event is marked, its activation status can be changed using the
marpa_g_completion_symbol_activate()
method. Note that, if a symbol is marked as a completion event symbol in the recognizer, its completion event can be deactivated and reactivated in the recognizer.Nulled rules and symbols will never cause completion events. Nullable symbols may be marked as completion event symbols, but this will have an effect only when the symbol is not nulled. Nulling symbols may be marked as completion event symbols, but no completion events will ever be generated for a nulling symbol. Note that this implies at no completion event will ever be generated at earleme 0, the start of parsing.
Success: On success, 1 if symbol sym_id is a completion event symbol after the call, 0 otherwise.
Failures: If sym_id is well-formed, but there is no such symbol, −1. If the grammar g is precomputed; or on other failure, −2.
Libmarpa can set up to generate an
MARPA_EVENT_SYMBOL_NULLED
event whenever the symbol is nulled. A symbol is said to be nulled when a zero length instance of that symbol is recognized.For nulled events to occur, the symbol must be marked as a nulled event symbol. The
marpa_g_symbol_is_nulled_event_set()
function marks symbol sym_id as a nulled event symbol if value is 1, and unmarks it it as a nulled event symbol if value is 0. Themarpa_g_symbol_is_nulled_event()
method returns the current value of the nulled event marking for symbol sym_id.Marking a nulled event sets its activation status to on. Unmarking a nulled event sets its activation status to off. The nulled event marking cannot be changed once the grammar is precomputed.
If a nulled event is marked, its activation status can be changed using the
marpa_g_nulled_symbol_activate()
method. Note that, if a symbol is marked as a nulled event symbol in the recognizer, its nulled event can be deactivated and reactivated in the recognizer.As a reminder, a symbol instance is a symbol at a specific location in the input, and with a specific length. Also, whenever a nulled symbol instance is recognized at a location, it is acceptable at that location, and vice versa.
When a symbol instance is recognized at a location, it will generate a nulled event or a prediction event, but never both. A symbol instance of zero length, when recognized at a location, generates a nulled event at that location, and does not generate a completion event. A symbol instance of non-zero length, when acceptable at a location, generates a completion event at that location, and does not generate a nulled event.
When a symbol instance is acceptable at a location, it will generate a nulled event or a prediction event, but never both. A symbol instance of zero length, when acceptable at a location, generates a nulled event at that location, and does not generate a prediction event. A symbol instance of non-zero length, when acceptable at a location, generates a prediction event at that location, and does not generate a nulled event.
While it is not possible for a symbol instance to generate both a nulled event and a completion event at a location, it is quite possible that a symbol might generate both kinds of event at that location. This is because multiple instances of the same symbol may be recognized at a given location, and these instances will have different lengths. If one instance is recognized at a given location as zero length and a second, non-zero-length, instance is recognized at the same location, the first will generate only nulled events, while the second will generate only completion events. For similar reasons, while a symbol instance will never generate both a null event and a prediction event at a location, multiple instances of the same symbol may do so.
Zero length derivations can be ambiguous. When a zero length symbol is recognized, all of its zero-length derivations are also considered to be recognized.
The
marpa_g_symbol_is_nulled_event_set()
method will mark a symbol as a nulled event symbol, even if the symbol is non-nullable. This is convenient, for example, for automatically generated grammars. Applications which wish to treat it as a failure if there is an attempt to mark a non-nullable symbol as a nulled event symbol, can check for this case using themarpa_g_symbol_is_nullable()
method.Success: On success, 1 if symbol sym_id is a nulled event symbol after the call, 0 otherwise.
Failures: If sym_id is well-formed, but there is no such symbol, −1. If the grammar g is precomputed; or on other failure, −2.
Libmarpa can be set up to generate a
MARPA_EVENT_SYMBOL_PREDICTED
event when a non-nulled symbol is predicted. A non-nulled symbol is said to be predicted when a instance of it is acceptable at the current earleme according to the grammar. Nulled symbols do not generate predictions.For predicted events to occur, the symbol must be marked as a predicted event symbol. The
marpa_g_symbol_is_predicted_event_set()
function marks symbol sym_id as a predicted event symbol if value is 1, and unmarks it it as a predicted event symbol if value is 0. Themarpa_g_symbol_is_predicted_event()
method returns the current value of the predicted event marking for symbol sym_id.Marking a prediction event sets its activation status to on. Unmarking a prediction event sets its activation status to off. The prediction event marking cannot be changed once the grammar is precomputed.
If a prediction event is marked, its activation status can be changed using the
marpa_g_prediction_symbol_activate()
method. Note that, if a symbol is marked as a prediction event symbol in the recognizer, its prediction event can be deactivated and reactivated in the recognizer.Success: On success, 1 if symbol sym_id is a predicted event symbol after the call, 0 otherwise.
Failures: If sym_id is well-formed, but there is no such symbol, −1. If the grammar g is precomputed; or on other failure, −2.
Precomputation is necessary for a recognizer to be generated from a grammar. On success,
marpa_g_precompute
returns a non-negative number to indicate that it precomputed the grammar without issues. On failure,marpa_g_precompute
returns −2.Precomputation may return one or more events, which may be queried using the
marpa_g_event()
method. At this point events only occur when failure is reported, and events always report issues. But application writers should expect future versions to have events which are reported on success, as well as events which do not represent issues.A
MARPA_EVENT_LOOP_RULES
event occurs when there are infinite loop rules (cycles) in the grammar. The presence of one or more of these will cause failure to be reported, but will not prevent the grammar from being precomputed.Each
MARPA_EVENT_COUNTED_NULLABLE
event is a symbol which is a nullable on the right hand side of a sequence rule — a “counted” symbol. The presence of one or more of these will cause failure to be reported, and will prevent the grammar from being precomputed. So that the programmer can fix several at once, these failures are delayed until events are created for all of the counted nullables.Each
MARPA_EVENT_NULLING_TERMINAL
event is a nulling symbol which is also flagged as a terminal. Since terminals cannot be of zero length, this is a logical impossibility. The presence of one or more of these will cause failure to be reported, and will prevent the grammar from being precomputed. So that the programmer can fix several at once, the failure is delayed until events are created for all of the counted nullables.Precomputation involves freezing and then thoroughly checking the grammar. Among the reasons for precomputation to fail are the following:
MARPA_ERR_NO_RULES
: The grammar has no rules.MARPA_ERR_NO_START_SYMBOL
: No start symbol was specified.MARPA_ERR_INVALID_START_SYMBOL
: A start symbol ID was specified, but it is not the ID of a valid symbol.MARPA_ERR_START_NOT_LHS
: The start symbol is not on the LHS of any rule.MARPA_ERR_UNPRODUCTIVE_START
: The start symbol is not productive.MARPA_ERR_COUNTED_NULLABLE
: A symbol on the RHS of a sequence rule is nullable. Libmarpa does not allow this.MARPA_ERR_NULLING_TERMINAL
: A terminal is also a nulling symbol. Libmarpa does not allow this.More details of these can be found under the description of the appropriate code. See External error codes.
marpa_g_precompute()
is unusual in that it is possible to treat one of its failures as “advisory”, and to proceed with parsing. Ifmarpa_g_precompute()
fails with an error code ofMARPA_ERR_GRAMMAR_HAS_CYCLE
, parsing can proceed, just as it typically would for success. The grammar will have been precomputed, as calling themarpa_g_is_precomputed()
method will confirm.Most applications, however, will want to simply treat failure with
MARPA_ERR_GRAMMAR_HAS_CYCLE
, as simply another failure, and fix the cycles before parsing. Cycles make a grammar infinitely ambiguous, and are considered useless in current practice. Cycles make processing the grammar less efficient, sometimes considerably so. Detection of cycles is returned as failure because that is by far the convenient thing to do for the vast majority of applications.Return value: On success, a non-negative number. On failure, −2.
Return value: On success, 1 if grammar g is already precomputed, 0 otherwise. On failure, −2.
This function allows the application to determine if grammar g has a cycle. As mentioned, most applications will want to treat these as fatal errors. To determine which rules are in the cycle,
marpa_g_rule_is_loop()
can be used.Return value: On success, 1 if the grammar has a cycle, 0 otherwise. On failure, −2.
An archetypal application uses a recognizer to read input.
To create a recognizer, use the marpa_r_new()
method.
When a recognizer is no longer in use, its memory can be freed
using the
marpa_r_unref()
method.
To make a recognizer ready for input,
use the marpa_r_start_input()
method.
The recognizer starts with its current earleme
at location 0.
To read a token at the current earleme,
use the marpa_r_alternative()
call.
To complete the processing of the current earleme,
and move forward to a new one,
use the marpa_r_earleme_complete()
call.
Creates a new recognizer. The reference count of the recognizer will be 1. The reference count of g, the base grammar, will be incremented by one.
Return value: On success, the newly created recognizer. If g is not precomputed, or on other failure,
NULL
.
Increases the reference count by 1. Not needed by most applications.
Return value: On success, the recognizer object, r. On failure,
NULL
.
Decreases the reference count by 1, destroying r once the reference count reaches zero. When r is destroyed, the reference count of its base grammar is decreased by one. If this takes the reference count of the base grammar to zero, it too is destroyed.
Makes r ready to accept input. The first Earley set, the one at earleme 0, will be completed during this call.
Because the call to
marpa_r_start_input()
completes an Earley set, it may generate events. For details about the events that may be generated during Earley set completion, see the description of themarpa_r_earleme_complete()
method.Return value: On success, a non-negative value. On failure, −2.
Reads a token into r. The token will start at the current earleme. Libmarpa allows tokens to be ambiguous, to be of variable length and to overlap. token_id is the symbol ID of the token, which must be a terminal. length is the length of the token.
value is an integer that represents the value of the token. In applications where the token's actual value is not an integer, it is expected that the application will use this value as a “virtual” value, perhaps finding the actual value by using value to index an array. value is not used inside Libmarpa — it is simply stored to be returned by the valuator as a convenience for the application. Some applications may prefer to track token values on their own, perhaps based on the earleme location and token_id, instead of using Libmarpa's token values.
A value of 0 is reserved for a now-deprecated feature. Do not use it. For more details on that feature, see the section Valued and unvalued symbols.
When
marpa_r_alternative()
is successful, the value of the furthest earleme is set to the greater of its value before the call, and current+length, where current is the value of the current earleme. The values of the current and latest earlemes are unchanged by calls tomarpa_r_alternative()
.Several error codes leave the recognizer in a fully recoverable state, allowing the application to retry the
marpa_r_alternative()
method. Retry is efficient, and quite useable as a parsing technique. The error code of primary interest from this point of view isMARPA_ERR_UNEXPECTED_TOKEN_ID
, which indicates that the token was not accepted because of its token ID. Retry after this condition is used in several applications, and is called “the Ruby Slippers technique”.The error codes
MARPA_ERR_DUPLICATE_TOKEN
,MARPA_ERR_NO_TOKEN_EXPECTED_HERE
andMARPA_ERR_INACCESSIBLE_TOKEN
also leave the recognizer in a fully recoverable state, and may also be useable for the Ruby Slippers or similar techniques. At this writing, the author knows of no applications which attempt to recover from these errors.Return value: On success,
MARPA_ERR_NONE
. On failure, some other error code.
This method does the final processing for the current earleme. It then advances the current earleme by one. Note that
marpa_r_earleme_complete()
may be called even when no tokens have been read at the current earleme — in the character-per-earleme input model, for example, tokens can span many characters and, if the input is unambiguous over that span, there will be no other tokens that start inside it.As mentioned,
marpa_r_earleme_complete()
always advances the current earleme, incrementing its value by 1. This means that value of the current earleme after the call will be the one plus the value of the earleme processed by the call tomarpa_r_earleme_complete()
. If any token was accepted at the earleme being processed,marpa_r_earleme_complete()
creates a new Earley set which will be the latest Earley set and, after the call, the latest earleme will be equal to the new current earleme. If no token was accepted at the earleme being processed, no Earley set is created, and the value of the latest earleme remains unchanged. The value of the furthest earleme is never changed by a call tomarpa_r_earleme_complete()
.During this method, one or more events may occur. On success, this function returns the number of events generated, but it is important to note that events may be created whether earleme completion fails or succeeds. When this method fails, the application must call
marpa_g_event()
if it wants to determine if any events occurred. Since the reason for failure to complete an earleme is often detailed in the events, applications that fail will often be at least as interested in the events as those that succeed.The
MARPA_EVENT_EARLEY_ITEM_THRESHOLD
event indicates that an application-settable threshold on the number of Earley items has been reached or exceeded. What this means depends on the application, but when the default threshold is exceeded, it means that it is very likely that the time and space resources consumed by the parse will prove excessive.A parse is “exhausted” when it can accept no more input. This can happen both on success and on failure. Note that the failure due to parse exhaustion only means failure at the current earleme. There may be successful parses at earlier earlemes.
If a parse is exhausted, but successful, an event with the event code
MARPA_EVENT_EXHAUSTED
occurs. Because the parse is exhausted, no input will be accepted at later earlemes. It is quite common for a parse to become exhausted when it succeeds. Many practical grammars are designed so that a successful parse cannot be extended.An exhausted parse may cause a failure, in which case
marpa_r_earleme_complete()
returns an error whose error code isMARPA_ERR_PARSE_EXHAUSTED
. For a parse to fail at an earleme due to exhaustion, it must be the case that no alternatives were accepted at that earleme. In fact, in the standard input model, a failure due to parse exhaustion occurs if and only if no alternatives were accepted at the current earleme.The circumstances under which failure due to parse exhaustion occurs are slightly more complicated when variable length tokens are in use. Informally, a parse will never fail due to exhaustion as long as it is possible that a token ending at some future earleme will continue the parse. More precisely, a call to
marpa_r_earleme_complete()
fails due to parse exhaustion if and only if, first, no alternatives were added at the current earleme and, second, that call left the current earleme equal to the furthest earleme.Return value: On success, the number of events generated. On failure, −2.
Marpa_Earleme
marpa_r_current_earleme (Marpa_Recognizer r)Return value: If input has started, the current earleme. If input has not started, −1. Always succeeds.
In the default, token-stream model, Earley set ID and earleme are always equal, but this is not the case in other input models. (The ID of an Earley set ID is also called its ordinal.) If there is no Earley set whose ID is set_id,
marpa_r_earleme()
fails. If set_id was negative, the error code is set toMARPA_ERR_INVALID_LOCATION
. If set_id is greater than the ordinal of the latest Earley set, the error code is set toMARPA_ERR_NO_EARLEY_SET_AT_LOCATION
.At this writing, there is no method for the inverse operation (conversion of an earleme to an Earley set ID). One consideration in writing such a method is that not all earlemes correspond to Earley sets. Applications that want to map earlemes to Earley sets will have no trouble if they are using the standard input model — the Earley set ID is always exactly equal to the earleme in that model. For other applications that want an earleme-to-ID mapping, the most general method is create an ID-to-earleme array using the
marpa_r_earleme()
method and invert it.Return value: On success, the earleme corresponding to Earley set set_id. On failure, −2.
Returns the integer value of earley_set. For more details, see the description of
marpa_r_earley_set_values()
.Return value: On success, the value of earley_set. On failure, −2.
If p_value is non-zero, sets the location pointed to by p_value to the integer value of the Earley set. Similarly, if p_pvalue is non-zero, sets the location pointed to by p_pvalue to the pointer value of the Earley set.
The “value” and “pointer” of an Earley set are an arbitrary integer and an arbitrary pointer that the application can use for its own purposes. In character-per-earleme input models, for example, the integer can be the codepoint of the current character. In a traditional token-per-earleme input model, they could be used to indicate the string value of the token – the pointer could point to the start of the string, and the integer could indicate its length.
The Earley set value and pointer can be set using the
marpa_r_latest_earley_set_values_set()
method. The Earley set integer value defaults to −1, and the pointer value defaults toNULL
.Return value: On success, returns a non-negative integer. On failure, returns −2.
unsigned int
marpa_r_furthest_earleme (Marpa_Recognizer r)Return value: On success, the furthest earleme. Always succeeds.
This method returns the Earley set ID (ordinal) of the latest Earley set. Applications that want the value of the latest earleme can convert this value using the
marpa_r_earleme()
method.Return value: On success, the ID of the latest Earley set. Always succeeds.
Sets the integer value of the latest Earley set. For more details, see the description of
marpa_r_latest_earley_set_values_set()
.Return value: On success, the new value of earley_set. On failure, −2.
Sets the integer and pointer value of the latest Earley set. For more about the “integer value” and “pointer value” of an Earley set, see the description of the
marpa_r_earley_set_values()
method.Return value: On success, returns a non-negative integer. On failure, returns −2.
Allows the user to deactivate and reactivate symbol completion events in the recognizer. If reactivate is zero, the event is deactivated. If reactivate is one, the event is activated.
Symbol completion events are active by default if the symbol was set up for completion events in the grammar. If a symbol was not set up for completion events in the grammar, symbol completion events are inactive by default and any attempt to change that is a fatal error.
Success cases: On success, the method returns the value of reactivate. The method succeeds trivially if the symbol is already set as indicated by reactivate.
Failure cases: If the active status of the completion event for sym_id cannot be set as indicated by reactivate, the method fails. On failure, −2 is returned.
These methods, respectively, set and query the Earley item warning threshold. The Earley item warning threshold is a number that is compared with the count of Earley items in each Earley set. When it is matched or exceeded, a
MARPA_EVENT_EARLEY_ITEM_THRESHOLD
event is created.If threshold is zero or less, an unlimited number of Earley items will be allowed without warning. This will rarely be what the user wants.
By default, Libmarpa calculates a value based on the grammar. The formula Libmarpa uses is the result of some experience, and most applications will be happy with it.
Return value: The value that the Earley item warning threshold has after the method call is finished. Always succeeds.
Sets the “expected symbol event bit” for symbol_id to value. A recognizer event is created whenever symbol symbol_id is expected at the current earleme. if and only if the expected symbol event bit for symbol_id is 1. The “expected symbol event bit” must be 1 or 0.
In this context, “expected” means “expected as a terminal”. Even if a symbol is predicted at the current earleme, if it is not acceptable as a terminal, it does not trigger an “expected symbol event”.
By default, the “expected symbol event bit” is 0. It is an error to attempt to set the “expected symbol event bit” to 1 for a nulling symbol, an inaccessible symbol, or an unproductive symbol.
Return value: The value of the event bit after the method call is finished. -2 if symbol_id is not the ID of a valid symbol; if it is the ID of an nulling, inaccessible for unproductive symbol; or on other failure.
A parser is “exhausted” if it cannot accept any more input. Both successful and failed parses can be exhausted. In many grammars, the parse is always exhausted as soon as it succeeds. Good parses may also exist at earlemes prior to the current one.
Return value: 1 if the parser is exhausted, 0 otherwise. Always succeeds.
Allows the user to deactivate and reactivate symbol nulled events in the recognizer. If boolean is zero, the event is deactivated. If boolean is one, the event is activated.
Symbol nulled events are active by default if the symbol was set up for nulled events in the grammar. If a symbol was not set up for nulled events in the grammar, symbol nulled events are inactive by default and any attempt to change that is a fatal error.
Success cases: On success, the method returns the value of boolean. The method succeeds trivially if the symbol is already set as indicated by boolean.
Failure cases: If the active status of the nulled event for sym_id cannot be set as indicated by boolean, the method fails. On failure, −2 is returned.
Allows the user to deactivate and reactivate symbol prediction events in the recognizer. If boolean is zero, the event is deactivated. If boolean is one, the event is activated.
Symbol prediction events are active by default if the symbol was set up for prediction events in the grammar. If a symbol was not set up for prediction events in the grammar, symbol prediction events are inactive by default and any attempt to change that is a fatal error.
Success cases: On success, the method returns the value of boolean. The method succeeds trivially if the symbol is already set as indicated by boolean.
Failure cases: If the active status of the prediction event for sym_id cannot be set as indicated by boolean, the method fails. On failure, −2 is returned.
Returns a list of the ID's of the symbols that are acceptable as tokens at the current earleme. buffer is expected to be large enough to hold the result. This is guaranteed to be the case if the buffer is large enough to hold a number of
Marpa_Symbol_ID
's that is greater than or equal to the number of symbols in the grammar.Return value: On success, the number of
Marpa_Symbol_ID
's in buffer. On failure, −2.
Return values on success: If symbol_id is the ID of a valid terminal symbol that is expected at the current earleme, a number greater than zero. If symbol_id is the ID of a valid terminal symbol that is not expected at the current earleme, or if symbol_id is the ID of a valid symbol that is not a terminal, zero.
Failure cases: Returns −2 on failure. It is a failure if symbol_id is not the ID of a valid symbol.
An important advantage of the Marpa algorithm is the ability to easily get full information about the state of the parse.
To start a progress report,
use the marpa_r_progress_report_start()
command.
Only one progress report can be in use at any one time.
To get the information in a progress report,
it is necessary to step through the progress report
items.
To get the data for the current progress report item,
and advance to the next one,
use the marpa_r_progress_item()
method.
To destroy a progress report,
freeing the memory it uses,
call the marpa_r_progress_report_finish()
method.
Resets the progress report. Assumes a report of the progress has already been initialized at some Earley set for recognizer r, with
marpa_r_progress_report_start()
. The reset progress report will be positioned before its first item.Return value: On success, a non-negative value. On failure, −2.
Initializes a report of the progress at Earley set set_id for recognizer r. If a progress report already exists, it is destroyed and its memory is freed. Initially, the progress report is positioned before its first item.
If no Earley set with ID set_id exists,
marpa_r_progress_report_start()
fails. The error code isMARPA_ERR_INVALID_LOCATION
if set_id is negative. The error code isMARPA_ERR_NO_EARLEY_SET_AT_LOCATION
if set_id is greater than the ID of the latest Earley set.Return value: On success, the number of report items available. If the recognizer has not been started; if set_id does not exist; or on other failure, −2.
Destroys the report of the progress at Earley set set_id for recognizer r, freeing the memory and other resources. It is often not necessary to call this method. Any previously existing progress report is destroyed automatically whenever a new progress report is started, and when the recognizer is destroyed.
Return value: −2 if no progress report has been started, or on other failure. On success, a non-negative value.
This method allows access to the data for the next item of a progress report. If there are no more progress report items, it returns −1 as a termination indicator and sets the error code to
MARPA_ERR_PROGRESS_REPORT_EXHAUSTED
. Either the termination indicator, or the item count returned bymarpa_r_progress_report_start()
, can be used to determine when the last item has been seen.On success, the dot position is returned in the location pointed to by the position argument, and the origin is returned in the location pointed to by the origin argument. On failure, the locations pointed to by the position and origin arguments are unchanged.
Return value: On success, the rule ID of the next progress report item. If there are no more progress report items, −1. If either the position or the origin argument is
NULL
, or on other failure, −2.
A bocage is structure containing the full set of parses found by processing the input according to the grammar. The bocage structure is new with Libmarpa, but is very similar in purpose to the more familar parse forests.
To create a bocage, use the marpa_b_new()
method.
When a bocage is no longer in use, its memory can be freed
using the
marpa_b_unref()
method.
Creates a new bocage object, with a reference count of 1. The reference count of its parent recognizer object, r, is increased by 1. If earley_set_ID is −1, the Earley set at the current earleme is used, if there is one.
If earley_set_ID is −1 and there is no Earley set at the current earleme; or if earley_set_ID is −1 and there is no parse ending at Earley set earley_set_ID,
marpa_b_new()
fails and the error code is set toMARPA_ERR_NO_PARSE
.Success return value: On success, the new bocage object. On failure,
NULL
.
Increases the reference count by 1. Not needed by most applications.
Return value: On success, b. On failure,
NULL
.
Decreases the reference count by 1, destroying b once the reference count reaches zero. When b is destroyed, the reference count of its parent recognizer is decreased by 1. If this takes the reference count of the parent recognizer to zero, it too is destroyed. If the parent recognizer is destroyed, the reference count of its base grammar is decreased by 1. If this takes the reference count of the base grammar to zero, it too is destroyed.
Returns an ambiguity metric. The metric is 1 is the parse is unambiguous. If the metric is 2 or greater, the parse is ambiguous. It was originally intended to have values greater than 2 be an cheaply computed estimate of the degree of ambiguity, but a satisfactory scheme for this has yet to be implemented.
Return value on success: 1 if the bocage is not for an ambiguous parse; 2 or greater if the bocage is for an ambiguous parse.
Failures: On failure, −2.
Return value on success: A number greater than or equal to 1 if the bocage is for a null parse; otherwise, 0.
Failures: On failure, −2.
Before iterating the parses in the bocage,
they must be ordered.
To create an ordering, use the marpa_o_new()
method.
When an ordering is no longer in use, its memory can be freed
using the
marpa_o_unref()
method.
An ordering is frozen once the first tree iterator is created using it. A frozen ordering cannot be changed.
As of this writing, the only methods to order parses are internal and undocumented. This is expected to change.
Creates a new ordering object, with a reference count of 1. The reference count of its parent bocage object, b, is increased by 1.
Return value: On success, the new ordering object. On failure,
NULL
.
Increases the reference count by 1. Not needed by most applications.
Return value: On success, o. On failure,
NULL
.
Decreases the reference count by 1, destroying o once the reference count reaches zero. Beginning with o's parent bocage, Libmarpa then proceeds up the chain of parent objects. Every time a child is destroyed, the reference count of its parent is decreased by 1. Every time the reference count of an object is decreased by 1, if that reference count is now zero, that object is destroyed. Libmarpa follows this chain of decrements and destructions as required, all the way back to the base grammar, if necessary.
Returns an ambiguity metric. The metric is 1 is the parse is unambiguous. If the metric is 2 or greater, the parse is ambiguous. It was originally intended to have values greater than 2 be an cheaply computed estimate of the degree of ambiguity, but a satisfactory scheme for this has yet to be implemented.
If the ordering is not already frozen, it will be frozen on return from
marpa_o_ambiguity_metric()
.marpa_o_ambiguity_metric()
is considered an “accessor”, because it is assumed that the ordering is frozen whenmarpa_o_ambiguity_metric()
is called.Return value on success: 1 if the ordering is not for an ambiguous parse; 2 or greater if the ordering is for an ambiguous parse.
Failures: On failure, −2.
Return value on success: A number greater than or equal to 1 if the ordering is for a null parse; otherwise, 0.
Failures: On failure, −2.
These methods, respectively, set and query the “high rank only” flag of ordering o. A flag of 1 indicates that, when ranking, all choices should be discarded except those of the highest rank. A flag of 0 indicates that no choices should be discarded on the basis of their rank.
A value of 1 is the default. The value of the “high rank only” flag has no effect unless ranking has been turned on using the
marpa_o_rank()
method.Return value: On success, the value of the “high rank only” flag after the call. On failure, −2.
By default, the ordering of parse trees is arbitrary. This method causes the ordering to be ranked according to the ranks of symbols and rules, the “null ranks high” flags of the rules, and the “high rank only” flag of the ordering. Once this method returns, the ordering is frozen.
Return value: On success, a non-negative value. On failure, −2.
Once the bocage has an ordering, the parses trees can be iterated. Marpa's parse tree iterators iterate the parse trees contained in a bocage object. In Libmarpa, “parse tree iterators” are usually just called trees.
To create a tree, use the marpa_t_new()
method.
A newly created tree iterator is positioned before the first parse tree.
When a tree iterator is no longer in use, its memory can be freed
using the
marpa_t_unref()
method.
To position a newly created tree iterator at the first parse tree,
use the marpa_t_next()
method.
Once the tree iterator is positioned at a parse tree,
the same marpa_t_next()
method is used
to position it to the next parse tree.
Creates a new tree iterator, with a reference count of 1. The reference count of its parent ordering object, o, is increased by 1.
When initialized, a tree iterator is positioned before the first parse tree. To position the tree iterator to the first parse, the application must call
marpa_t_next()
.Return value: On success, a newly created tree. On failure,
NULL
.
Increases the reference count by 1. Not needed by most applications.
Return value: On success, t. On failure,
NULL
.
Decreases the reference count by 1, destroying t once the reference count reaches zero. Beginning with t's parent ordering, Libmarpa then proceeds up the chain of parent objects. Every time a child is destroyed, the reference count of its parent is decreased by 1. Every time the reference count of an object is decreased by 1, if that reference count is now zero, that object is destroyed. Libmarpa follows this chain of decrements and destructions as required, all the way back to the base grammar, if necessary.
Positions t at the next parse tree in the iteration. Tree iterators are initialized to the position before the first parse tree, so this method must be called before creating a valuator from a tree.
If a tree iterator is positioned after the last parse, the tree is said to be “exhausted”. A tree iterator for a bocage with no parse trees is considered to be “exhausted” when initialized. If the tree iterator is exhausted,
marpa_t_next()
returns −1 as a termination indicator, and sets the error code toMARPA_ERR_TREE_EXHAUSTED
.Return value: On success, a non-negative value. If the tree iterator is exhausted, −1. On failure, −2.
The parse counter counts the number of parse trees traversed so far. The count includes the current iteration of the tree, so that a value of 0 indicates that the tree iterator is at its initialized position, before the first parse tree.
Return value: The number of parses traversed so far. Always succeeds.
The archetypal application needs
a value object (or valuator) to produce
the value of the parse.
To create a valuator, use the marpa_v_new()
method.
When a valuator is no longer in use, its memory can be freed
using the
marpa_v_unref()
method.
The application is required to maintain the stack,
and the application is also required to implement
most of the semantics, including the evaluation
of rules.
Libmarpa's valuator provides instructions to
the application on how to manipulate the stack.
To iterate through this series of instructions,
use the marpa_v_step()
method.
marpa_v_step()
returns the type
of step.
Most step types have values associated with them.
To access these values use the methods
described in the section Basic step accessors.
How to perform the steps is described in
the sections
How to use the valuator
and Stepping through the valuator.
Libmarpa's valuator provides the application with “steps”, which are instructions for stack manipulation. Libmarpa itself does not maintain a stack. This leaves the upper layer in total control of the stack and the values which are placed on it.
As example may make this clearer. Suppose the evalution is at a place in the parse tree where an addition is being performed. Libmarpa does not know that the operation is an addition. It will tell the application that rule number R is to be applied to the arguments at stack locations N and N+1, and that the result is to placed in stack location N.
In this system the application keeps track of the semantics for all rules, so it looks up rule R and determines that it is an addition. The application can do this by using R as an index into an array of callbacks, or by any other method it chooses. Let's assume a callback implements the semantics for rule R. Libmarpa has told the application that two arguments are available for this operation, and that they are at locations N and N+1 in the stack. They might be the numbers 42 and 711. So the callback is called with its two arguments, and produces a return value, let's say, 753. Libmarpa has told the application that the result belongs at location N in the stack, so the application writes 753 to location N.
Since Libmarpa knows nothing about the semantics, the operation for rule R could be string concatenation instead of addition. Or, if it is addition, it could allow for its arguments to be floating point or complex numbers. Since the application maintains the stack, it is up to the application whether the stack contains integers, strings, complex numbers, or polymorphic objects which are capable of being any of these things and more.
Step-driven valuation hides Libmarpa's grammar rewrites from the application, and is quite efficient. Libmarpa knows which rules are sequences. Libmarpa optimizes stack manipulations based on this knowledge. Long sequences are very common in practical grammars. For these, the stack manipulations suggested by Libmarpa's step-driven valuator will be significantly faster than the traditional stack evaluation algorithm.
Step-driven evalution has another advantage. To illustrate this, consider what is a very common case: The semantics are implemented in a higher-level language, using callbacks. If Libmarpa did not use step-driven valuation, it would need to provide for this case. But for generality, Libmarpa would have to deal in C callbacks. Therefore, a middle layer would have to create C language wrappers for the callbacks in the higher level language.
The implementation that results is this: The higher level language would need to wrap each callback in C. When calling Libmarpa, it would pass the wrappered callback. Libmarpa would then need to call the C language “wrappered” callback. Next, the wrapper would call the higher-level language callback. The return value, which would be data native to the higher-level language, would need to be passed to the C language wrapper, which will need to make arrangements for it to be based back to the higher-level language when appropriate.
A setup like this is not terribly efficient. And exception handling across language boundaries would be very tricky. But neither of these is the worst problem.
Callbacks are hard to debug. Wrappered callbacks are even worse. Calls made across language boundaries are harder yet to debug. In the system described above, by the time a return value is finally consumed, a language boundary will have been crossed four times.
How do programmers deal with difficulties like this? Usually, by doing the absolute minimum possible in the callbacks. A horrific debugging enviroment can become a manageable one if there is next to no code to be debugged. And this can be accomplished by doing as much as possible in pre- and post-processing.
In essence, callbacks force applications to do most of the programming via side effects. One need not be a functional programming purist to find this a very undesirable style of design to force on an application. But the ability to debug can make the difference between code that does work and code that does not. Unfairly or not, code is rarely considered well-designed when it does not work.
So, while step-driven valuation seems a roundabout approach, it is simpler and more direct than the likely alternatives. And there is something to be said for pushing semantics up to the higher levels — they can be expected to know more about it.
These advantages of step-driven valuation are strictly in the context of a low-level interface. The author is under no illusion that direct use of Libmarpa's valuator will be found satisfactory by most application programmers, even those using the C language. The author certainly avoids using step-driven valuation directly. Libmarpa's valuator is intended to be used via an upper layer, one which does know about semantics.
This section discusses in detail the requirements for maintaining the stack. In some cases, such as implementation using a Perl array, fulfilling these requirements is trivial. Perl auto-extends its arrays, and initializes the element values, on every read or write. For the C programmer, things are not quite so easy.
In this section, we will assume a C90 or C99 standard-conformant C application. This assumption is convenient on two grounds. First, this will be the intended use for many readers. Second, standard-conformant C is a “worst case”. Any issue faced by a programmer of another environment is likely to also be one that must be solved by the C programmer.
Libmarpa often optimizes away unnecessary stack writes to stack locations. When it does so, it will not necessarily optimize away all reads to that stack location. This means that a location's first access, as suggested by the Libmarpa step instructions, may be a read. This possibility requires a special awareness from the C programmer, as discussed in the sections Sizing the stack and Initializing locations in the stack.
In the discussions in this document, stack locations are non-negative integers. The bottom of the stack is location 0. In moving from the bottom of the stack to the top, the numbers increase. Stack location Y is said to be “greater” than stack location X if stack location Y is closer to the top of stack than location X, and therefore stack locations are considered greater or lesser if the integers that represent them are greater or lesser. Another way to state that a stack location Y is greater (lesser) than stack location X is to say that a stack location Y is later (earlier) than stack location X.
If an implementation applies Libmarpa's step instructions literally, using a physical stack, it must make sure the stack is large enough. Specifically, the application must do the following
MARPA_STEP_TOKEN
steps,
ensure that location marpa_v_result(v)
exists.
MARPA_STEP_NULLING_SYMBOL
steps,
ensure that location marpa_v_result(v)
exists.
MARPA_STEP_RULE
steps,
ensure that stack locations from marpa_v_arg_0(v)
to marpa_v_arg_n(v)
exist.
Three aspects of these requirements deserve special mention.
First,
note that the requirement for a
MARPA_STEP_RULE
is that the application
size the stack to include the arguments to be
read.
Because stack writes may be optimized away,
an application,
when reading,
cannot assume
that the stack was
sized appropriately by a prior write.
The first access to a new stack location may be
a read.
Second,
note that there is no explicit requirement that
the application size the stack to include the
location for the result of the
MARPA_STEP_RULE
step.
An application is allowed to assume that
result will go into one of the locations
that were read.
Third, special note should be made of the requirement that location 0 exist. By convention, the parse result resides in location 0 of the stack. Because of potential optimizations, an application cannot assume that it will receive a Libmarpa step instruction that either reads from or writes to location 0.
Write optimizations also creates issues for implementations which require data to be initialized before reading. Every fully standard-conforming C application is such an implementation. Both C90 and C99 allow “trap values”, and therefore conforming applications must be prepared for an uninitialized location to contain one of those. Reading a trap value may cause an abend. (It is safe, in standard-conforming C, to write to a location containing a trap value.)
The requirement that locations be initialized before reading occurs in other implementations. Any implementation that has a “universe” of “safe” values, may require special precautions. The required precautions may amount to a need to initialize “uninitialized” values. A practical example might be an implementation that expects all locations to contain a pointer which it can safely indirect from. In such implementations, just as in standard-conformant C, every stack location needs to be initialized before being read.
Due to write optimizations, an application cannot rely on Libmarpa's step instructions to initialize every stack location before its first read. One way to safely deal with the initialization of stack locations, is to do all of the following:
Applications which try to optimize out some of these initializations need to be aware that an application can never assume that activity in the stack is safely “beyond” an uninitialized location. Libmarpa steps often revisit earlier sections of the stack, and these revisits may include reads of previously unvisited stack locations.
Creates a new valuator. The parent object of the new valuator will be the tree iterator t, and the reference count of the new valuator will be 1. The reference count of t is increased by 1.
The parent tree iterator is “paused”, so that the tree iterator cannot move on to a new parse tree until the valuator is destroyed. Many valuators of the same parse tree can exist at once. A tree iterator is “unpaused” when all of the valuators of that tree iterator are destroyed.
Return value: On success, the newly created valuator. On failure,
NULL
.
Increases the reference count by 1. Not needed by most applications.
Return value: On success, v. On failure,
NULL
.
Decreases the reference count by 1, destroying v once the reference count reaches zero. Beginning with v's parent tree, Libmarpa then proceeds up the chain of parent objects. Every time a child is destroyed, the reference count of its parent is decreased by 1. Every time the reference count of an object is decreased by 1, if that reference count is now zero, that object is destroyed. Libmarpa follows this chain of decrements and destructions as required, all the way back to the base grammar, if necessary.
This method “steps through” the valuator. The return value is a
Marpa_Step_Type
, an integer which indicates the type of step. How the application is expected to act on each step is described below. See Valuator steps by type. When the iteration through the steps is finished,marpa_v_step
returnsMARPA_STEP_INACTIVE
.Return value: On success, the type of the step to be performed. This will always be a non-negative number. On failure, −2.
The semantics of a rule should be performed. The application can find the value of the rule's children in the stack locations from
marpa_v_arg_0(v)
tomarpa_v_arg_n(v)
. The semantics for the rule whose ID ismarpa_v_rule(v)
should be executed on these child values, and the result placed inmarpa_v_result(v)
. In the case of aMARPA_STEP_RULE
step, the stack location ofmarpa_v_result(v)
is guaranteed to be equal tomarpa_v_arg_0(v)
.
The semantics of a non-null token should be performed. The application's value for the token whose ID is
marpa_v_token(v)
should be placed in stack locationmarpa_v_result(v)
. Its value according to Libmarpa will be inmarpa_v_token_value(v)
.
The semantics for a nulling symbol should be performed. The ID of the symbol is
marpa_v_symbol(v)
and its value should be placed in stack locationmarpa_v_result(v)
.
The valuator has gone through all of its steps and is now inactive. The value of the parse will be in stack location 0. Because of optimizations, it is possible for valuator to immediately became inactive —
MARPA_STEP_INACTIVE
could be both the first and last step.
The valuator is new and has yet to go through any steps.
These step types are reserved for internal purposes.
The basic step accessors are so called because their information is basic to the stack manipulation. The basic step accessors are implemented as macros. They always succeed.
For a
MARPA_STEP_RULE
step, returns the stack location where the value of first child can be found.
For a
MARPA_STEP_RULE
step, returns the stack location where the value of the last child can be found.
For
MARPA_STEP_RULE
,MARPA_STEP_TOKEN
, andMARPA_STEP_NULLING_SYMBOL
steps, returns the stack location where the result of the semantics should be placed.
For the
MARPA_STEP_RULE
step, returns the ID of the rule.
Returns the current step type:
MARPA_STEP_TOKEN
,MARPA_STEP_RULE
, etc. Usually not needed since this is also the return value ofmarpa_v_step()
.
For the
MARPA_STEP_NULLING_SYMBOL
step, returns the ID of the symbol. The value returned is the same as that returned by themarpa_v_token()
macro.
For the
MARPA_STEP_TOKEN
step, returns the ID of the token. The value returned is the same as that returned by themarpa_v_symbol()
macro.
For the
MARPA_STEP_TOKEN
step, returns the integer which is (or which represents) the value of the token.
This section contains the step accessors that are not basic to stack manipulation, but which provide other useful information about the parse. These step accessors are implemented as macros.
All of these accessors always succeed, but if called when they are irrelevant they return an unspecified value. In this context, an “unspecified value” is a value that is either −1 or the ID of a valid Earley set, but which is otherwise unpredictable.
Return value: If the current step type is
MARPA_STEP_RULE
, the Earley Set ordinal where the rule ends. If the current step type isMARPA_STEP_TOKEN
orMARPA_STEP_NULLING_SYMBOL
, the Earley Set ordinal where the symbol ends. If the current step type is anything else, an unspecified value.
Return value: If the current step type is
MARPA_STEP_RULE
, the Earley Set ordinal where the rule begins. If the current step type is anything else, an unspecified value.
Return value: If the current step type is
MARPA_STEP_TOKEN
orMARPA_STEP_NULLING_SYMBOL
, the Earley Set ordinal where the token begins. If the current step type is anything else, an unspecified value.
Events are generated by the
marpa_g_precompute()
,
marpa_r_earleme_complete()
,
and
marpa_r_start_input()
methods.
The methods are called event-active.
Event-active methods always clear all previous events,
so that after an event-active method the only events
available
will be those generated by that method.
Events are volatile, and it is expected that events will be queried immediately after the method that generated them. Note especially that multiple recognizers using the same base grammar overwrite each other's events.
To find out how many events were generated by the last
event-active method,
use the marpa_g_event_count
method.
To query a specific event,
use the marpa_g_event
and
marpa_g_event_value
methods.
On success, the type of the ix'th event is returned and the data for the ix'th event is placed in the location pointed to by event.
Event indexes are in sequence. Valid events will be in the range from 0 to n, where n is one less than the event count. The event count can be queried using the
marpa_g_event_count()
method.Return value: On success, the type of event ix. If there is no ix'th event, if ix is negative, or on other failure, −2. On failure, the locations pointed to by event are not changed.
Return value: On success, the number of events. On failure, −2.
This macro provides access to the “value” of the event. The semantics of the value varies according to the type of the event, and is described in the section on event codes. See Event codes.
Applications should never see this event. Event value: Undefined. Suggested message: "No event".
A nullable symbol is either the separator for, or the right hand side of, a sequence. Event value: The ID of the symbol. Suggested message: "This symbol is a counted nullable".
This event indicates the current Earley item count exceeded a user-settable threshold. Event value: The current Earley item count. Suggested message: "Too many Earley items".
The parse is exhausted. Event value: Undefined. Suggested message: "Recognizer is exhausted".
One or more rules are loop rules — rules that are part of a cycle. Cycles are pathological cases of recursion, in which the same symbol string derives itself a potentially infinite number of times. Nonetheless, Marpa parses in the presence of these, and it is up to the application to treat these as fatal errors, something they almost always will wish to do. Event value: The count of loop rules. Suggested message: "Grammar contains a infinite loop".
A nulling symbol is also a terminal. Event value: The ID of the symbol. Suggested message: "This symbol is a nulling terminal".
The recognizer can be set to generate an event a symbol is completed using its
marpa_g_symbol_is_completion_event_set()
method. (A symbol is "completed" if and only if any rule with that symbol as its LHS is completed.) This event code indicates that one of those events occurred. Event value: The ID of the completed symbol. Suggested message: "Completed symbol".
The recognizer can be set to generate an event when a symbol is expected as a terminal, using its
marpa_r_expected_symbol_event_set()
method. Note that this event only triggers if the symbol is expected as a terminal. Predicted symbols which are not expected as terminals do not trigger this event. This event code indicates that one of those events occurred. Event value: The ID of the expected symbol. Suggested message: "Expecting symbol".
The recognizer can be set to generate an event when a symbol is nulled – that is, recognized as a zero-length symbol. To set an nulled symbol event, use the recognizer's
marpa_r_nulled_symbol_event_set()
method. This event code indicates that a nulled symbol event occurred. Event value: The ID of the nulled symbol. Suggested message: "Symbol was nulled".
The recognizer can be set to generate an event when a symbol is predicted. To set an predicted symbol event, use the recognizer's
marpa_r_predicted_symbol_event_set()
method. Unlike theMARPA_EVENT_SYMBOL_EXPECTED
event, theMARPA_EVENT_SYMBOL_PREDICTED
event triggers for predictions of both non-terminals and terminals. This event code indicates that a predicted symbol event occurred. Event value: The ID of the predicted symbol. Suggested message: "Symbol was predicted".
When a method fails, this method allows the application to read the error code. p_error_string is reserved for use by the internals. Applications should set it to
NULL
.Return value: The last error code from a Libmarpa method. Always succeeds.
Sets the error code to
MARPA_ERR_NONE
. Not often used, but now and then it can be useful to force the error code to a known state.Return value:
MARPA_ERR_NONE
. Always succeeds.
The number of error codes. All error codes, whether internal or external, will be integers, non-negative but strictly less than
MARPA_ERRCODE_COUNT
.
This section lists the external error codes. These are the only error codes that users of the Libmarpa external interface should ever see. Internal error codes are in their own section. See Internal error codes.
No error condition. The error code is initialized to this value. Methods which do not result in failure sometimes reset the error code to
MARPA_ERR_NONE
. Numeric value: 0. Suggested message: "No error".
A separator was specified for a sequence rule, but its ID was not that of a valid symbol. Numeric value: 6. Suggested message: "Separator has invalid symbol ID".
A tree iterator is positioned before the first tree, and it was specified in a context where that is not allowed. A newly created tree is positioned before the first tree. To position a newly created tree iterator to the first tree use the
marpa_t_next()
method. Numeric value: 91. Suggested message: "Tree iterator is before first tree".
A “counted” symbol was found that is also a nullable symbol. A “counted” symbol is one that appears on the RHS of a sequence rule. If a symbol is nullable, counting its occurrences becomes difficult. Questions of definition and problems of implementation arise. At a minimum, a sequence with counted nullables would be wildly ambigious.
Sequence rules are simply an optimized shorthand for rules that can also be written in ordinary BNF. If the equivalent of a sequence of nullables is really what your application needs, nothing in Libmarpa prevents you from specifying that sequence with ordinary BNF rules.
Numeric value: 8. Suggested message: "Nullable symbol on RHS of a sequence rule".
This error indicates an attempt to add a BNF rule which is a duplicate of a BNF rule already in the grammar. Two BNF rules are considered duplicates if
- Both rules have the same left hand symbol, and
- Both rules have the same right hand symbols in the same order.
Duplication of sequence rules, and duplication between BNF rules and sequence rules, is dealt with by requiring that the LHS of a sequence rule not be the LHS of any other rule.
Numeric value: 11. Suggested message: "Duplicate rule".
This error indicates an attempt to add a duplicate token. A token is a duplicate if one already read at the same earleme has the same symbol ID and the same length. Numeric value: 12. Suggested message: "Duplicate token".
This error code indicates that an implementation-defined limit on the number of Earley items per Earley set was exceedeed. This limit is different from the Earley item warning threshold, an optional limit on the number of Earley items in an Earley set, which can be set by the application.
The implementation defined-limit is very large, at least 500,000,000 earlemes. An application is unlikely ever to see this error. Libmarpa's use of memory would almost certainly exceed the implementation's limits before it occurred. Numeric value: 13. Suggested message: "Maximum number of Earley items exceeded".
A negative event index was specified. That is not allowed. Numeric value: 15. Suggested message: "Negative event index".
An non-negative event index was specified, but there is no event at that index. Since the events are in sequence, this means it was too large. Numeric value: 16. Suggested message: "No event at that index".
The grammar has a cycle — one or more loop rules. This is a recoverable error, although most applications will want to treat it as fatal. For more see the description of marpa_g_precompute. Numeric value: 17. Suggested message: "Grammar has cycle".
This is an internal error, and indicates that Libmarpa was wrongly built. Libmarpa was compiled with headers which do not match the rest of the code. The solution is to find a correctly built Libmarpa. Numeric value: 98. Suggested message: "Internal error: Libmarpa was built incorrectly"
The Libmarpa base grammar is in a "not ok" state. Currently, the only way this can happen is if Libmarpa memory is being overwritten. Numeric value: 29. Suggested message: "Marpa is in a not OK state".
This error code indicates that the token symbol is an inaccessible symbol — one which cannot be reached from the start symbol.
Since the inaccessibility of a symbol is a property of the grammar, this error code typically indicates an application error. Nevertheless, a retry at this location, using another token ID, may succeed. At this writing, the author knows of no uses of this technique.
Numeric value: 18. Suggested message: "Token symbol is inaccessible".
A function was called that takes a boolean argument, but the value of that argument was not either 0 or 1. Numeric value: 22. Suggested message: "Argument is not boolean".
The location (Earley set ID) is not valid. It may be invalid for one of two reasons:
- It is negative, and it is being used as the argument to a method for which that negative value does not have a special meaning.
- It is after the latest Earley set.
For users of input models other than the standard one, the term “location”, as used in association with this error code, means Earley set ID or Earley set ordinal. In the standard input model, this will always be identical with Libmarpa's other idea of location, the earleme.
Numeric value: 25. Suggested message: "Location is not valid".
A start symbol was specified, but its symbol ID is not that of a valid symbol. Numeric value: 27. Suggested message: "Specified start symbol is not valid".
A method was called with an invalid assertion ID. This is a assertion ID which not only does not exist, but cannot exist. Currently that means its value is less than zero. Numeric value: 96. Suggested message: "Assertion ID is malformed".
A method was called with an invalid rule ID. This is a rule ID which not only does not exist, but cannot exist. Currently that means its value is less than zero. Numeric value: 26. Suggested message: "Rule ID is malformed".
A method was called with an invalid symbol ID. This is a symbol ID which not only does not exist, but cannot exist. Currently that means its value is less than zero. Numeric value: 28. Suggested message: "Symbol ID is malformed".
There was a mismatch in the major version number between the requested version of libmarpa, and the actual one. Numeric value: 30. Suggested message: "Libmarpa major version number is a mismatch".
There was a mismatch in the micro version number between the requested version of libmarpa, and the actual one. Numeric value: 31. Suggested message: "Libmarpa micro version number is a mismatch".
There was a mismatch in the minor version number between the requested version of libmarpa, and the actual one. Numeric value: 32. Suggested message: "Libmarpa minor version number is a mismatch".
A non-negative Earley set ID (also called an Earley set ordinal) was specified, but there is no corresponding Earley set. Since the Earley set ordinals are in sequence, this means that the specified ID is greater than that of the latest Earley set. Numeric value: 39. Suggested message: "Earley set ID is after latest Earley set".
The grammar is not precomputed, and attempt was made to do something with it that is not allowed for unprecomputed grammars. For example, a recognizer cannot be created from a grammar until it is precomputed. Numeric value: 34. Suggested message: "This grammar is not precomputed".
The application attempted to create a bocage from a recognizer without a parse. Applications will often want to treat this as a soft error. Numeric value: 41. Suggested message: "No parse".
A grammar which has no rules is being used in a way that is not allowed. Usually the problem is that the user is trying to precompute the grammar. Numeric value: 42. Suggested message: "This grammar does not have any rules".
The grammar has no start symbol, and an attempt was made to perform an operation which requires one. Usually the problem is that the user is trying to precompute the grammar. Numeric value: 43. Suggested message: "This grammar has no start symbol".
A method was called with an assertion ID which is well-formed, but the assertion does not exist. Numeric value: 97. Suggested message: "No assertion with this ID exists".
A method was called with a rule ID which is well-formed, but the rule does not exist. Numeric value: 89. Suggested message: "No rule with this ID exists".
A method was called with a symbol ID which is well-formed, but the symbol does not exist. Numeric value: 90. Suggested message: "No symbol with this ID exists".
This error code indicates that no tokens at all were expected at this earleme location. This can only happen in alternative input models.
Typically, this indicates an application programming error. Retrying input at this location will always fail. But if the application is able to leave this earleme empty, a retry at a later location, using this or another token, may succeed. At this writing, the author knows of no uses of this technique.
Numeric value: 44. Suggested message: "No token is expected at this earleme location".
This error occurs in situations where a rule is required to be a sequence, and indicates that the rule of interest is, in fact, not a sequence.
Numeric value: 99. Suggested message: "Rule is not a sequence".
Marpa does not allow a symbol to be both nulling and a terminal. Numeric value: 49. Suggested message: "A symbol is both terminal and nulling".
The Marpa order object has been frozen. If a Marpa order object is frozen, it cannot be changed.
Multiple tree iterators can share a Marpa order object, but that order object is frozen after the first tree iterator is created from it. Applications can order an bocage in many ways, but they must do so by creating multiple order objects.
Numeric value: 50. Suggested message: "The ordering is frozen".
The parse is exhausted. Numeric value: 53. Suggested message: "The parse is exhausted".
The parse is too long. The limit on the length of a parse is implementation dependent, but it is very large, at least 500,000,000 earlemes.
This error code is unlikely in the standard input model. Almost certainly memory would be exceeded before it could occur. If an application sees this error, it almost certainly using one of the non-standard input models.
Most often this messsage will occur because of a request to add a single extremely long token, perhaps as a result of an application error. But it is also possible this error condition will occur after the input of a large number of long tokens.
Numeric value: 54. Suggested message: "This input would make the parse too long".
In a method which takes pointers as arguments, one of the pointer arguments is
NULL
, in a case where that is not allowed. One such method ismarpa_r_progress_item()
. Numeric value: 56. Suggested message: "An argument is null when it should not be".
An attempt was made to use a precomputed grammar in a way that is not allowed. Often this is an attempt to change the grammar. Nearly every change to a grammar after precomputation invalidates the precomputation, and is therefore not allowed. Numeric value: 57. Suggested message: "This grammar is precomputed".
No recognizer progress report is currently active, and an action has been attempted which is inconsistent with that. One such action would be a
marpa_r_progress_item()
call. Numeric value: 59. Suggested message: "No progress report has been started".
The progress report is “exhausted” — all its items have been iterated through. Numeric value: 58. Suggested message: "The progress report is exhausted".
A symbol or rule rank was specified which was less than an implementation-defined minimum. Implementations will always allow at least those ranks in the range between −134,217,727 and 134,217,727. Numeric value: 85. Suggested message: "Rule or symbol rank too low".
A symbol or rule rank was specified which was greater than an implementation-defined maximum. Implementations will always allow at least those ranks in the range between −134,217,727 and 134,217,727. Numeric value: 86. Suggested message: "Rule or symbol rank too high".
The recognizer is “inconsistent”, usually because the user has rejected one or more rules or terminals, and has not yet called the
marpa_r_consistent()
method. Numeric value: 95. Suggested message: "The recognizer is inconsistent.
The recognizer is not accepting input, and the application has attempted something that is inconsistent with that fact. Numeric value: 60. Suggested message: "The recognizer is not accepting input".
The recognizer has not been started. and the application has attempted something that is inconsistent with that fact. Numeric value: 61. Suggested message: "The recognizer has not been started".
The recognizer has been started. and the application has attempted something that is inconsistent with that fact. Numeric value: 62. Suggested message: "The recognizer has been started".
The index of a RHS symbol was specified, and it was negative. That is not allowed. Numeric value: 63. Suggested message: "RHS index cannot be negative".
A non-negative index of RHS symbol was specified, but there is no symbol at that index. Since the indexes are in sequence, this means the index was greater than or equal to the rule length. Numeric value: 64. Suggested message: "RHS index must be less than rule length".
An attempt was made to add a rule with too many right hand side symbols. The limit on the RHS symbol count is implementation dependent, but it is very large, at least 500,000,000 symbols. This is far beyond what is required in any current practical grammar. An application with rules of this length is almost certain to run into memory and other limits. Numeric value: 65. Suggested message: "The RHS is too long".
The LHS of a sequence rule cannot be the LHS of any other rule, whether a sequence rule or a BNF rule. An attempt was made to violate this restriction. Numeric value: 66. Suggested message: "LHS of sequence rule would not be unique".
The start symbol is not on the LHS on any rule. That means it could never match any possible input, not even the null string. Presumably, an error in writing the grammar. Numeric value: 73. Suggested message: "Start symbol not on LHS of any rule".
An attempt was made to use a symbol in a way that requires it to be set up for completion events, but the symbol was not set set up for completion events. The archetypal case is an attempt to activate completion events for the symbol in the recognizer. The archetypal case is an attempt to activate a completion event in the recognizer for a symbol that is not set up as a completion event. Numeric value: 92. Suggested message: "Symbol is not set up for completion events".
An attempt was made to use a symbol in a way that requires it to be set up for nulled events, but the symbol was not set set up for nulled events. The archetypal case is an attempt to activate a nulled events in the recognizer for a symbol that is not set up as a nulled event. Numeric value: 93. Suggested message: "Symbol is not set up for nulled events".
An attempt was made to use a symbol in a way that requires it to be set up for predictino events, but the symbol was not set set up for predictino events. The archetypal case is an attempt to activate a prediction event in the recognizer for a symbol that is not set up as a prediction event. Numeric value: 94. Suggested message: "Symbol is not set up for prediction events".
Unvalued symbols are a deprecated Marpa feature, which may be avoided with the
marpa_g_force_valued
method. An unvalued symbol may take on any value, and therefore a symbol which is unvalued at some points cannot safely to be used to contain a value at others. This error indicates that such an unsafe use is being attempted. Numeric value: 74. Suggested message: "Symbol is treated both as valued and unvalued".
An attempt was made to change the terminal status of a symbol to a different value after it was locked. Numeric value: 75. Suggested message: "The terminal status of the symbol is locked".
A token was specified whose symbol ID is not a terminal. Numeric value: 76. Suggested message: "Token symbol must be a terminal".
A token length was specified which is less than or equal to zero. Zero-length tokens are not allowed in Libmarpa. Numeric value: 77. Suggested message: "Token length must greater than zero".
The token length is too long. The limit on the length of a token is implementation dependent, but it is at least 500,000,000 earlemes. An application using a token that long is almost certain to run into some other limit. Numeric value: 78. Suggested message: "Token is too long".
A Libmarpa parse tree iterator is “exhausted”, that is, it has no more parses. Numeric value: 79. Suggested message: "Tree iterator is exhausted".
A Libmarpa tree is “paused” and an operation was attempted which is inconsistent with that fact. Typically, this operation will be a call of the
marpa_t_next()
method. Numeric value: 80. Suggested message: "Tree iterator is paused".
An attempt was made to read a token where a token with that symbol ID is not expected. This message can also occur when an attempt is made to read a token at a location where no token is expected. Numeric value: 81. Suggested message: "Unexpected token".
The start symbol is unproductive. That means it could never match any possible input, not even the null string. Presumably, an error in writing the grammar. Numeric value: 82. Suggested message: "Unproductive start symbol".
The valuator is inactive in a context where that should not be the case. Numeric value: 83. Suggested message: "Valuator inactive".
Unvalued symbols are a deprecated Marpa feature, which may be avoided with the
marpa_g_force_valued
method. This error code indicates that the valued status of a symbol is locked, and an attempt was made to change it to a status different from the current one. Numeric value: 84. Suggested message: "The valued status of the symbol is locked".
An attempt was made to do something with a nulling symbol that is not allowed. For example, the ID of a nulling symbol cannot be an argument to
marpa_r_expected_symbol_event_set
— because it is not possible to create an “expected symbol” event for a nulling symbol. Numeric value: 87. Suggested message: "Symbol is nulling".
An attempt was made to do something with an unused symbol that is not allowed. An “unused” symbol is a inaccessible or unproductive symbol. For example, the ID of a unused symbol cannot be an argument to
marpa_r_expected_symbol_event_set
— because it is not possible to create an “expected symbol” event for an unused symbol. Numeric value: 88. Suggested message: "Symbol is not used".
An internal error code may be one of two things: First, it can be an error code which arises from an internal Libmarpa programming issue (in other words, something happening in the code that was not supposed to be able to happen.) Second, it can be an error code which only occurs when a method from Libmarpa's internal interface is used. Both kinds of internal error message share one common trait — users of the Libmarpa's external interface should never see them.
Internal error messages require someone with knowledge of the Libmarpa internals to follow up on them. They usually do not have descriptions or suggested messages.
"Development" errors were used heavily during Libmarpa's development, when it was not yet clear how precisely to classify every error condition. Unless they are using a developer's version, users of the external interface should never see development errors.
Development errors have an error string associated with them. The error string is a short 7-bit ASCII error string which describes the error. Numeric value: 9. Suggested message: "Development error, see string".
The AHFA ID was invalid. There are no AHFAs any more, so this message should not occur. Numeric value: 20.
The AHM ID was invalid. The term “AIMID” is a legacy of earlier implementations and must be kept for backward compatibility. Numeric value: 21.
Design apologetics are throughout this document, dispersed among sections where it is hoped that they motivate the description or discussion. The notes in this section did not find a home elsewhere.
Marpa is an aggressively multi-pass algorithm. Marpa achieves its efficiency, not in spite of making multiple passes over the data, but because of it. Marpa regularly substitutes two fast O(n) passes for a single O(n log n) pass. Marpa's proliferation of time objects is in keeping with its multi-pass approach.
Bocage objects come at no cost, even for unambiguous parses, because the same pass which creates the bocage also deals with other issues which are of major significance for unambiguous parses. It is the post-processing of the bocage pass that enables Marpa to do both left- and right-recursion in linear time.
Of the various objects, the best case for elimination is of the ordering object. In many cases, the ordering is trivial. Either the parse is unambiguous, or the application does not care about the order in which parses are returned. But while it would be easy to add an option to bypass creation of an ordering object, there is little to be gained from it. When the ordering is trivial, its overhead is very small — essentially a handful of subroutine calls. Many orderings accomplish nothing, but these cost next to nothing.
Tree objects come at minimal cost to unambiguous grammars, because the same pass that allows iteration through multiple parse trees does the tree traversal. This eliminates much of the work that otherwise would need to be done in the valuation time object. In the current implement, the valuation time object needs only to step through a sequence already determined in the tree iterator.
As the name suggests, the choice was made to implement numbered objects as integers, and not as pointers. In standard-conformant C, integers can be safely checked for validity, while pointers cannot.
There are efficiency tradeoffs between pointers and integers but they are complicated, and they go both ways. Pointers can be faster, but integers can be used as indexes into more than one data structure. Which is actually faster depends on the design. Integers allow for a more flexible design, so that once the choice is settled on, careful programming can make them a win, possibly a very big one.
The approach taken in Libmarpa was to settle, from the outset, on integers as the implementation for numbered objects, and to optimize on that basis. The author concedes that it is possible that others redoing Libmarpa from scratch might find that pointers are faster. But the author is confident that they will also discover, on modern architectures, that the lack of safe validity checking is far too high a price to pay for the difference in speed.
Marpa's idea in losing the sharp division between terminals and non-terminals is that the distinction, while helpful for proving theorems, is not essential in practice. LHS symbols in the input might be useful for “short circuiting” the rules in which they occur. This may prove helpful in debugging, or have other applications.
However, it also can be useful, for checking input validity as well as for efficiency, to follow tradition and distinguish non-terminals from terminals. For this reason, the traditional behavior is the default in Libmarpa.
The methods of this section are not in the external interface, because they have not been adequately tested. Their fate is uncertain. Users should regard these methods as unsupported.
These methods, respectively, set and query the default rank of the grammar. When a grammar is created, the default rank is 0. When rules and symbols are created, their rank is the default rank of the grammar.
Changing the grammar's default rank does not affect those rules and symbols already created, only those that will be created. This means that the grammar's default rank can be used to, in effect, assign ranks to groups of rules and symbols. Applications may find this behavior useful.
Return value: On success, returns the rank after the call, and sets the error code to
MARPA_ERR_NONE
. On failure, returns −2, and sets the error code to an appropriate value, which will never beMARPA_ERR_NONE
. Note that when the rank is −2, the error code is the only way to distinguish success from failure. The error code can be determined by using themarpa_g_error()
call.
These methods, respectively, set and query the rank of a symbol sym_id. When sym_id is created, its rank initialized to the default rank of the grammar.
Return value: On success, returns the rank after the call, and sets the error code to
MARPA_ERR_NONE
. On failure, returns −2, and sets the error code to an appropriate value, which will never beMARPA_ERR_NONE
. Note that when the rank is −2, the error code is the only way to distinguish success from failure. The error code can be determined by using themarpa_g_error()
call.
On success, returns previous default value of the assertion.
Changes default value to default_value. On success, returns previous default value of the assertion.
Marpa allows an application to “change its mind” about a parse, rejected rule previously recognized or predicted, and terminals previously scanned. The methods in this section provide that capability.
Libmarpa symbols can have values, which is the traditional way of doing semantics. Libmarpa also allows symbols to be unvalued. An unvalued symbol is one whose value is unpredictable from instance to instance. If a symbol is unvalued, we sometimes say that it has “whatever” semantics.
Situations where the semantics can tolerate unvalued symbols are surprisingly frequent. For example, the top-level of many languages is a series of major units, all of whose semantics are typically accomplished via side effects. The compiler is typically indifferent to the actual value produced by these major units, and tracking them is a waste of time. Similarly, the value of the separators in a list is typically ignored.
Rules are unvalued if and only if their LHS symbols are unvalued. When rules and symbols are unvalued, Libmarpa optimizes their evaluation.
It is in principle unsafe to check the value of a symbol if it can be unvalued. For this reason, once a symbol has been treated as valued, Libmarpa marks it as valued. Similarly, once a symbol has been treated as unvalued, Libmarpa marks it as unvalued. Once marked, a symbol's valued status is locked and cannot be changed later.
The valued status of terminals is marked the first time they are read. The valued status of LHS symbols must be explicitly marked by the application when initializing the valuator — this is Libmarpa's equivalent of registering a callback.
LHS terminals are disabled by default. If allowed, the user should be aware that the valued status of a LHS terminal will be locked in the recognizer if it is used as a terminal, and the symbol's use as a rule LHS in the valuator must be consistent with the recognizer's marking.
Marpa reports an error when a symbol's use conflicts with its locked valued status. Doing so usually saves the programmer some tricky debugging further down the road.
These methods, respectively, set and query the “valued status” of a symbol. Once set to a value with the
marpa_g_symbol_is_valued_set()
method, the valued status of a symbol is “locked” at that value. It cannot thereafter be changed. Subsequent calls tomarpa_g_symbol_is_valued_set()
for the same sym_id will fail, leaving sym_id's valued status unchanged, unless value is the same as the locked-in value.Return value: On success, 1 if the symbol symbol_id is valued after the call, 0 if not. If the valued status is locked and value is different from the current status, −2. If value is not 0 or 1; or on other failure, −2.
By default, Libmarpa's valuator objects
assume that
non-terminal symbols have
no semantics.
The archetypal application will need to register
symbols that contain semantics.
The primary method for doing this is
marpa_v_symbol_is_valued()
.
Applications will typically register semantics by rule,
and these applications will find
the marpa_v_rule_is_valued()
method more convenient.
These methods, respectively, set and query the valued status of symbol sym_id.
marpa_v_symbol_is_valued_set()