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Author image Jeffrey Kegler
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Marpa::Evaluator - Marpa Evaluator Objects


    my $fail_offset = $lexer->text('2-0*3+1');
    if ( $fail_offset >= 0 ) {
        Marpa::exception("Parse failed at offset $fail_offset");

    my $evaler = Marpa::Evaluator->new( { recognizer => $recce } );
    Marpa::exception('Parse failed') if not $evaler;

    my $i = 0;
    while ( defined( my $value = $evaler->value() ) ) {
        my $value = ${$value};
        Test::More::ok( $expected_value{$value}, "Value $i (unspecified order)" );
        delete $expected_value{$value};
    } ## end while ( defined( my $value = $evaler->value() ) )


Parses are found and evaluated by Marpa's evaluator objects. Evaluators are created with the new constructor, which requires a Marpa recognizer object as an argument.

Marpa allows ambiguous parses, so evaluator objects are iterators. Iteration is performed with the value method, which returns a reference to the value of the next parse. Often only the first parse is needed, in which case the value method can be called just once.

By default, the new constructor clones the recognizer, so that multiple evaluators do not interfere with each other.

Null Values

A "null value" is the value used for a symbol when it is nulled in a parse. By default, the null value is a Perl undefined. The default null value is a Marpa option (default_null_value) and can be reset.

Each symbol can have its own null symbol value. The null symbol value for any symbol is calculated using the null symbol action. The null symbol action for a symbol is the action specified for the empty rule with that symbol on its left hand side. The null symbol action is not a rule action. It's a property of the symbol, and applies whenever the symbol is nulled, even when the symbol's empty rule is not involved.

For example, in MDL, the following says that whenever the symbol A is nulled, its value should be a string that says it is missing.

    A: . q{'A is missing'}.

Null symbol actions are evaluated differently from rule actions. Null symbol actions are run at evaluator creation time and the value of the result at that point becomes fixed as the null symbol value. This is not the case with rule actions. During the creation of the evaluator object, rule actions are compiled into closures. During parse evaluation, whenever a node for that rule needs its value recalculated, the compiled rule closure is run. A compiled rule closure can produce a different value every time it runs.

I treat null symbol actions differently for efficiency. They have no child values, and a fixed value is usually what is wanted. If you want to calculate a symbol's null value with a closure run at parse evaluation time, the null symbol action can return a reference to a closure. Rules with that nullable symbol in their right hand side can then be set up to run that closure.

Evaluating Null Derivations

A null derivation may consist of many steps and may contain many symbols. Marpa's rule is that the value of a null derivation is the null symbol value of the highest null symbol in that derivation. This section describes in detail how a parse is evaluated, focusing on what happens when nulled symbols are involved.

The first step in evaluating a parse is to determine which nodes count for the purpose of evaluation, and which do not. Marpa follows these principles:

  1. The start node always counts.

  2. Nodes count if they derive a non-empty sentence.

  3. All other nodes do not count.

  4. In evaluating a parse, Marpa uses only nodes that count.

These are all consequences of the principles above:

  1. The value of null derivation is the value of the highest null symbol in it.

  2. A nulled node counts only if it is the start node.

  3. The value of a null parse is the null value of the start symbol.

If you think some of the rules or symbols represented by nodes that don't count are important in your grammar, Marpa can probably accommodate your ideas. First, for every nullable symbol, determine how to calculate the value which your semantics produces when that nullable symbol is a "highest null symbol". If it's a constant, write a null action for that symbol which returns that constant. If your semantics do not produce a constant value by evaluator creation time, write a null action which returns a reference to a closure and arrange to have that closure run by the parent node.


Suppose a grammar has these rules


    S: A, Y. q{ $_[0] . ", but " . $_[1] }. # Call me the start rule
    note: you can also call me Rule 0.

    A: . q{'A is missing'}. # Call me Rule 1

    A: B, C. q{"I'm sometimes null and sometimes not"}. # Call me Rule 2

    B: . q{'B is missing'}. # Call me Rule 3

    C: . q{'C is missing'}. # Call me Rule 4

    C: Y.  q{'C matches Y'}. # Call me Rule 5

    Y: /Z/. q{'Zorro was here'}. # Call me Rule 6

In the above MDL, the Perl 5 regex "/Z/" occurs on the rhs of Rule 6. Where a regex is on the rhs of a rule, MDL internally creates a terminal symbol to match that regex in the input text. In this example, the MDL internal terminal symbol that matches input text using the regex /Z/ will be called Z.

If the input text is the Perl 5 string "Z", the derivation is as follows:

    S -> A Y      (Rule 0)
      -> A "Z"    (Y produces "Z", by Rule 6)
      -> B C "Z"  (A produces B C, by Rule 2)
      -> B "Z"    (C produces the empty string, by Rule 4)
      -> "Z"      (B produces the empty string, by Rule 3)

The parse tree can be described as follows:

    Node 0 (root): S (2 children, nodes 1 and 4)
        Node 1: A (2 children, nodes 2 and 3)
            Node 2: B (matches empty string)
            Node 3: C (matches empty string)
        Node 4: Y (1 child, node 5)
            Node 5: "Z" (terminal node)

Here's a table showing, for each node, its lhs symbol, the sentence it derives, and its value.

                        Symbol      Sentence     Value

    Node 0:                S         "Z"         "A is missing, but Zorro is here"
        Node 1:            A         empty       "A is missing"
            Node 2:        B         empty       No value
            Node 3:        C         empty       No value
        Node 4:            Y         "Z"         "Zorro was here"
            Node 5:        -         "Z"         "Z"

In this derivation, nodes 1, 2 and 3 derive the empty sentence. None of them are the start node so that none of them count.

Nodes 0, 4 and 5 all derive the same non-empty sentence, Z, so they all count. Node 0 is the start node, so it would have counted in any case.

Since node 5 is a terminal node, it's value comes from the lexer. Where the lexing is done with a Perl 5 regex, the value will be the Perl 5 string that the regex matched. In this case it's the string "Z".

Node 4 is not nulled, so it is evaluated normally, using the rule it represents. That is rule 6. The action for rule 6 returns "Zorro was here", so that is the value of node 4. Node 4 has a child node, node 5, but rule 6's action pays no attention to child values. The action for each rule is free to use or not use child values.

Nodes 1, 2 and 3 don't count and will all remain unevaluated. The only rule left to be evaluated is node 0, the start node. It is not nulled, so its value is calculated using the action for the rule it represents (rule 0).

Rule 0's action uses the values of its child nodes. There are two child nodes and their values are elements 0 and 1 in the @_ array of the action. The child value represented by the symbol Y, $_[1], comes from node 4. From the table above, we can see that that value was "Zorro was here".

The first child value is represented by the symbol A, which is nulled. For nulled symbols, we must use the null symbol value. Null symbol values for each symbol can be explicitly set by specifying an rule action for an empty rule with that symbol as its lhs. For symbol A, this was done in Rule 1. Rule 1's action evaluates to the Perl 5 string "A is missing".

Even though rule 1's action plays a role in calculating the value of this parse, rule 1 is not actually used in the derivation. No node in the derivation represents rule 1. Rule 1's action is used because it is the null symbol action for the symbol A.

Now that we have both child values, we can use rule 0's action to calculate the value of node 0. That value is "A is missing, but Zorro was here", This becomes the value of S, rule 0's left hand side symbol and the start symbol of the grammar. A parse has the value of its start symbol, so "A is missing, but Zorro was here" is also the value of the parse.


The new constructor requires a recognizer object to be one of its arguments. By default, the new constructor clones the recognizer object. This is done so that evaluators do not interfere with each other by modifying the same data. Cloning is the default behavior, and is always safe.

While safe, cloning does impose an overhead in memory and time. This can be avoided by using the clone option with the new constructor. Not cloning is safe if you know that the recognizer object will not be shared by another evaluator. You must also be sure that the underlying grammar object is not being shared by multiple recognizers.

It is very common for a Marpa program to have a simple structure, where no more than one recognizer is created from any grammar, and no more than one evaluator is created from any recognizer. When this is the case, cloning is unnecessary.



    my $evaler = Marpa::Evaluator->new(
      { recognizer => $recce }

    my $evaler = Marpa::Evaluator->new( {
        recce => $recce,
        end => $location,
        clone => 0,
    } );

The new method's one, required, argument is a hash reference of named arguments. The new method either returns a new evaluator object or throws an exception. The recognizer option is required, Its value must be a recognizer object which has finished recognizing a text. The recce option is a synonym for the the recognizer option.

By default, parsing ends at the default end of parsing, which was set in the recognizer. If an end option is specified, it will be used as the number of the earleme at which to end parsing.

If the clone argument is set to 1, new clones the recognizer object, so that multiple evaluators do not interfere with each other's data. This is the default and is always safe. If clone is set to 0, the evaluator will work directly with the recognizer object which was its argument. See above for more detail.

Marpa options can also be named arguments to new. For these, see Marpa::Doc::Options.


    $evaler->set( { trace_values => 1 } );

The set method takes as its one, required, argument a reference to a hash of named arguments. It allows Marpa options to be specified for an evaluator object. Relatively few Marpa options are not available at evaluation time. The options which are available are mainly those which control evaluation time tracing. set either returns true or throws an exception.


    my $result = $evaler->value();

Iterates the evaluator object, returning a reference to the value of the next parse. If there are no more parses, returns undefined. Successful parses may evaluate to a Perl 5 undefined, which the value method will return as a reference to an undefined. Failures are thrown as exceptions.

When the order of parses is important, it may be manipulated by assigning priorities to the rules and terminals. If a symbol can both match a token and derive a rule, the token match always takes priority. Otherwise the parse order is implementation dependent.

A failed parse does not always show up as an exhausted parse in the recognizer. Just because the recognizer was active when it was used to create the evaluator, does not mean that the input matches the grammar. If it does not match, there will be no parses and the value method will return undefined the first time it is called.


See the support section in the main module.


Jeffrey Kegler


Copyright 2007 - 2009 Jeffrey Kegler

This program is free software; you can redistribute it and/or modify it under the same terms as Perl 5.10.0.