NAME
Marpa::R2::Grammar - Marpa grammars
SYNOPSIS
my $grammar = Marpa::R2::Grammar->new(
{ start => 'Expression',
actions => 'My_Actions',
default_action => 'first_arg',
rules => [
{ lhs => 'Expression', rhs => [qw/Term/] },
{ lhs => 'Term', rhs => [qw/Factor/] },
{ lhs => 'Factor', rhs => [qw/Number/] },
{ lhs => 'Term', rhs => [qw/Term Add Term/], action => 'do_add' },
{ lhs => 'Factor',
rhs => [qw/Factor Multiply Factor/],
action => 'do_multiply'
},
],
}
);
$grammar->precompute();DESCRIPTION
To create a Marpa grammar object, use the new method. Rules and symbols may be specified when the grammar is created.
To change a Marpa grammar object, use the set method. New rules may be added until a grammar is precomputed.
A grammar cannot be used for parsing until it is precomputed. To precompute a Marpa grammar object, use the precompute method. After precomputation, no new rules may added and most other changes are forbidden.
Symbol names
Symbol names that end in right square brackets are reserved for Marpa's internal use. Any other valid Perl string is an acceptable symbol name.
Terminal symbols
Marpa defines a terminal as a symbol which is valid as an input token symbol. By default, the terminals are those symbols who do not appear on the LHS of any rule.
Marpa will allow any non-nulling symbol to be a terminal, even those which appear on the LHS of one or more rules. To allow (or disallow) use of a symbol as a terminal, the application can use the terminals named argument, and the terminal property. An attempt to use a nulling symbol as a terminal is a fatal error.
Sequence rules
It is very common in a grammar for one symbol to produce a repeating sequence. Marpa allows a shorthand for this: sequence rules. The RHS of a sequence rule will be repeated, as specified by the min rule property. In sequence rules the RHS must always be one symbol in length, and that symbol may not be a nullable symbol.
A rule is a sequence rule if the min rule property is defined. min can be 0 or 1, and specifies the minimum number of times that the sequence is allowed to repeat. As of this writing, the maximum number of repetitions is always infinite.
{ lhs => 'sequence', rhs => ['item'], min => 0 }A min of zero indicates a sequence that repeats zero or more times. This is the equivalent of using the star quantifier ("*") in the standard regular expression notation.
{ lhs => 'sequence', rhs => ['item'], min => 1 }A min of one indicates a sequence that repeats one or more times. This is the equivalent of using the plus quantifier ("+") in the standard regular expression notation.
Sequences can have a separator, specified with the separator rule property. By default, separation is Perl-style: trailing separators are allowed. In "proper" separation, a separator must actually separate two sequence items and therefore is not allowed after the last item of a sequence. If you prefer "proper" separation, you can set the proper rule property.
Advantages of sequence rules
You are never forced to use sequence rules, but it's usually better if you do. When a sequence is written as a sequence rule, Marpa optimizes it.
When a sequence is written using non-sequence rules, the semantics typically wind up being spread over two or three Perl closures. The semantic action for a sequence rule is a single Perl closure. Putting the semantics into a single Perl closure often results in simpler and more natural code. See the section on sequences in the semantics document.
Caveats
Marpa throws an exception if you try to use a nullable symbol as the right hand side of a sequence rule, or as the separator for a sequence rule. The ban on nullables in sequences only applies to sequences when they are written using sequence rules. Nothing prevents you from specifying a sequence of nullables using non-sequence rules. But usually there is no good reason to do this, and sequences of nullables can be highly ambiguous which, for efficiency reasons, makes them a good thing to avoid.
To keep things simple, the right hand side of a sequence rule must be a single symbol. Of course, applications will often want to repeat sequences of multiple symbols. That is easy to do indirectly:
{ lhs => 'sequence', rhs => [qw(item)], min => 0 },
{ lhs => 'item', rhs => [qw(part1 part2)], },CONSTRUCTOR
new
my $grammar = Marpa::R2::Grammar->new(
{ start => 'Expression',
actions => 'My_Actions',
default_action => 'first_arg',
rules => [
{ lhs => 'Expression', rhs => [qw/Term/] },
{ lhs => 'Term', rhs => [qw/Factor/] },
{ lhs => 'Factor', rhs => [qw/Number/] },
{ lhs => 'Term', rhs => [qw/Term Add Term/], action => 'do_add' },
{ lhs => 'Factor',
rhs => [qw/Factor Multiply Factor/],
action => 'do_multiply'
},
],
}
);Marpa::R2::Grammar::new returns a new Marpa grammar object or throws an exception. The arguments to Marpa::R2::Grammar::new are references to hashes of named arguments. In each key/value pair of this hash, the hash key is the argument name and the hash value is the value of the named argument. The available named arguments are described below.
MUTATORS
precompute
$grammar->precompute();The precompute method compiles data structures that the recognizer will need. It returns the grammar object or throws an exception.
set
$grammar->set( { trace_file_handle => $trace_fh } );The arguments to the set method are references to hashes of named arguments. The available named arguments are described below. set either returns true or throws an exception.
ACCESSORS
check_terminal
Returns a Perl true when its argument is the name of a terminal symbol. Otherwise, returns a Perl false. Not often needed, but a lexer may find this the most convenient way to determine if a symbol is a terminal.
TRACE ACCESSORS
show_problems
print $grammar->show_problems()
or die "print failed: $ERRNO";Usually the application does not call this method directly. Returns a string describing any serious but non-fatal problems a grammar had in the precomputation phase. A serious problem is one that will prevent parsing. Warnings are not serious problems in this sense. If there were no serious problems, returns a string saying so. This method is not useful before precomputation.
In Marpa, most serious grammar problems are not immediately thrown as exceptions. This is because there can be a number of serious problems in a grammar, particularly one that is large or in an early draft. If each serious problem caused an immediate exception, the user would have to fix them one at a time -- very tedious.
The recognizer throws an exception when the user attempts to create a parse from a grammar with serious problems. When that happens, the string returned by show_problems is part of the error message.
show_rules
print $grammar->show_rules()
or die "print failed: $ERRNO";Returns a string listing the rules. Each rule is shown with comments which indicate rule properties. show_rules is useful in debugging grammars.
Marpa does extensive rewriting of its grammars, and both the original rules and the rewritten rules appear in the show_rules list. When a rule is rewritten, the original rule is often not used. In that case, "!used" will be one of the comments for the original rule. The "!used" comment also marks rules not used for reasons other than rewrites. For example, inaccessible and unproductive rules are also marked "!used".
The "discard_sep" comment indicates that the rule discards separators This is only relevant in sequence rules. Other comments indicate whether rules were nullable, unproductive, inaccessible, or empty.
show_symbols
print $grammar->show_symbols()
or die "print failed: $ERRNO";Returns a string listing the symbols, along with comments indicating whether they were terminal, nulling, nullable, unproductive or inaccessible. Useful for debugging grammars.
NAMED ARGUMENTS
action_object
The action_object named argument specifies a class name to be used in resolving action names to Perl closures. A new constructor must be defined in the action_object package. It will be used to create the per-parse-tree variables. Details are in the document on semantics.
actions
actions => 'My_Actions',The actions named argument specifies the Perl package that Marpa will use when resolving action names to Perl closures. If both an actions named argument and an action_object named argument are specified, the package from the actions named argument is the only one used to resolve action names. The actions package is treated only as a package, and not as a class. Any new constructor in the actions package is ignored. Details are given in the document on semantics.
default_action
default_action => 'first_arg',The default_action named argument specifies the value action name for rules without an action property. Details are given in the document on semantics.
default_empty_action
The default_empty_action named argument specifies the action for empty (zero length) rules which have no action specified explicitly. Details are given in the document on semantics.
inaccessible_ok
The value must be a reference to an array of symbol names. By default, Marpa warns if a symbol is inaccessible, but the warning is suppressed for any symbol named in the array. Setting the inaccessible_ok named argument after grammar precomputation is useless, and itself results in a warning.
Inaccessible symbols are symbols which cannot be derived from the start symbol, and which therefore can never be part of a successful parse. Inaccessible symbols often indicate errors in grammar design. But a user may have plans for these symbols, may wish to keep them as notes, or may simply wish to deal with them later.
infinite_action
Takes as its value a string specifying what Marpa should do if it discovers that its grammar is infinitely ambiguous. The value must be one of "fatal", "warn" or "quiet". A grammar is infinitely ambiguous if there is some input for which it produces an endless number of parses.
If the value is "fatal", Marpa throws an exception when it encounters an infinitely ambiguous grammar. This is the default and will usually be what the user wants. In most cases, an infinitely ambiguous grammar is simply a mistake.
"quiet" indicates that the user wants to allow infinitely ambiguous grammars. "warn" indicates that the user wants to allow infinitely ambiguous grammars, but wants a warning message to be printed to the trace file handle.
rules
The value of the rules named argument is a reference to an array of rule descriptors. The rules named argument may be specified multiple times, adding new rules to the grammar each time. New rules may be added until the grammar is precomputed. The format of rule descriptors is explained below.
start
start => 'Expression',The value of the start named argument must be a symbol name. It will be used as the start symbol for the grammar. The start named argument is required.
symbols
The value of the symbols named arguments must be a reference to a hash. In each key/value pair of this hash, the hash key is the symbol property name and the hash value is the symbol descriptor. Symbol descriptors are described below.
Note that the value of symbols named argument is a hash, but the value of the rules named argument is an array. This is because symbol names make convenient hash keys. For rules, there is no equally natural choice for a hash key.
terminals
The value of the terminals named argument must be a reference to an array of symbol names. All the symbols in the array will be allowed as terminals. See the discussion of terminals above.
trace_file_handle
The value is a file handle. Trace output and warning messages go to the trace file handle. By default the trace file handle is STDERR.
unproductive_ok
The value must be a reference to an array of symbol names. By default, Marpa warns if a symbol is unproductive, but the warning is suppressed for any symbol named in the array. Setting the unproductive_ok named argument after grammar precomputation is useless, and itself results in a warning.
Unproductive symbols are symbols which can never derive a sentence. (A sentence is a string of zero or more terminals.) That means that unproductive symbols can never be part of a successful parse. Unproductive symbols often indicate errors in grammar design. But a user may have plans for these symbols, may wish to keep them as notes, or may simply wish to deal with them later.
warnings
The value is a boolean. Warnings are written to the trace file handle. By default, warnings are on. Usually, an application will want to leave them on. If warnings are turned off, turning them back on after grammar precomputation is useless, and itself results in a warning.
RULE DESCRIPTORS
rules => [
{ lhs => 'Expression', rhs => [qw/Term/] },
{ lhs => 'Term', rhs => [qw/Factor/] },
{ lhs => 'Factor', rhs => [qw/Number/] },
{ lhs => 'Term', rhs => [qw/Term Add Term/], action => 'do_add' },
{ lhs => 'Factor',
rhs => [qw/Factor Multiply Factor/],
action => 'do_multiply'
},
],Rule descriptors as hashes
The long form descriptor of a rule is a reference to a hash of rule properties. In each key/value pair of this hash, the hash key is the rule property name and the hash value is the value of that property.
action
The value of the action rule property is a string which specifies the semantics for the rule. For details, see the document on semantics.
The semantics of nulling symbols are dealt with on a per-symbol basis, rather than a per-rule basis. For this reason the action rule property is useless for empty rules. An exception is thrown if an action property is defined for an empty rule.
keep
Separators in sequence rules are usually not semantically significant. By default, Marpa throws away separators during parse tree traversal and before node evaluation time, so that the semantic actions do not see the separators.
If the value of the keep rule property is a Perl true, Marpa keeps separators. This allows the semantic actions to examine them. The downside is that the work of distinguishing sequence separators from sequence items is pushed into the semantic actions. For details about the semantics, see the document on semantics.
lhs
The value of the lhs rule property must be a string containing the name of the rule's left hand side symbol. Every Marpa rule must have a left hand side symbol.
The lhs plays a role in the semantics. If no action rule property is defined, Marpa looks in either the grammar's actions package or the grammar's action_object for a Perl closure that has the name of the lhs symbol. For details, see the document on semantics.
min
min must be 0, 1, or undefined. If min is 0 or 1, the rule is a sequence rule. If min is undefined, the rule is an ordinary BNF rule.
Only one symbol, called the sequence item, is allowed on the right hand side of a sequence rule. The sequence item may not be a nullable symbol. The input will be required to match the sequence item at least min times and will be allowed to match the sequence item an unlimited number of times.
null_ranking
null_ranking is ignored unless the recognizer's ranking_method named argument is set to something other than its default. The null_ranking named argument allows the application to control the order in which rules with nullable symbols are returned by the value method. Such rules can match the same input in several ways depending on which symbols are nulled. These different ways of nulling symbols in a rule are called its null variants.
If null_ranking is undefined, the order of the null variants will be arbitrary. This is the default, and is acceptable to most applications. For details on using the null_ranking named argument, see the document on parse order.
proper
By default, sequence rules with separators allow trailing separators, Perl-style. If the proper rule property is a Perl true, "proper" separation is enforced. In proper separation, separation must actually separate sequence items, and trailing separators are not allowed.
rank
rank is ignored unless the recognizer's ranking_method named argument is set to something other than its default. rank, when specified, must be an integer. It may be negative. rank is 0 by default. For details on using the rank named argument, see the document on parse order.
rhs
The value of the rhs property is a reference to an array of strings containing the names of the rule's right hand symbols, in order. This array may be zero length, in which case this is an empty rule -- a rule with no symbols on the right hand side. A rule is also empty if there is no rhs specifier in its descriptor.
separator
Any sequence rule may have a separator defined. The value must be a symbol name. By default, Marpa allows trailing separators. This is the usual style in Perl. The separator must not be a nullable symbol.
Rule descriptors as arrays
rules => [
[ 'E', [qw/E Add E/], 'do_add' ],
[ 'E', [qw/E Multiply E/], 'do_multiply' ],
[ 'E', [qw/Number/], ],
],Rule descriptors may be given in "short form" -- as a reference to an array. The elements of the array, in order, are the lhs property, the rhs property, and the action property. The last two are optional. Omission of an optional property in a short form descriptor has the same effect that omitting the same optional property would have in the long form.
Duplicate rules
Marpa throws an exception if a duplicate rule is added. Two BNF rules are considered duplicates if
Both rules have the same left hand symbol.
Both rules have the same right hand symbols in the same order.
Sequence rules are even more restricted. The LHS of a sequence rule may not be the LHS of another sequence rule. The LHS of a sequence rule also may not be the LHS of any BNF rule.
This restriction on the LHS of sequence rules is intended to make the definition of duplicate rules intuitive and their detection easy. It does not limit the expressiveness of Marpa grammars, because it is very easy to work around. One workaround to create an intermediate rule of length one, whose RHS is the sequence LHS symbol. The LHS of the intermediate rule can then be used, without restriction, as the LHS of other rules.
COPYRIGHT AND LICENSE
Copyright 2012 Jeffrey Kegler
This file is part of Marpa::R2. Marpa::R2 is free software: you can
redistribute it and/or modify it under the terms of the GNU Lesser
General Public License as published by the Free Software Foundation,
either version 3 of the License, or (at your option) any later version.
Marpa::R2 is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser
General Public License along with Marpa::R2. If not, see
http://www.gnu.org/licenses/.