Re^2: First steps with Marpa::R2 and BNF

no sorry GrandFather but a big thanks

Anyway i dont understand your reply. I removed :default ::= action => [name,values] and is this the root cause of the "bogus" array (it makes sense to know which name is passing around but I consider it misleading in the sysnopsis).

I read in the doc you linked:

> The "::first" action indicates that the value of a rule is to be the value of its first child, that is, the value corresponding to the first symbol of the rule's RHS.

But I cannot see how your addition of action => ::first can fix it. Given I removed the :default ::= action I get the same results with or without ::first


Dice_Expression ::= Simple_Dice
            |Dice_with_modifier_x
            |Dice_with_modifier_r

do_simple_roll received: ({}, 4, "d", 6)
        Rolled : 4
        Rolled : 3
        Rolled : 6
        Rolled : 2
do_simple_roll returning: { die_type => "1d6", rolls => [4, 3, 6, 2] }
modifier_r received: ({}, { die_type => "1d6", rolls => [4, 3, 6, 2] }
+, "r", 1)



Dice_Expression ::= Simple_Dice action => ::first 
                    |Dice_with_modifier_x action => ::first
                    |Dice_with_modifier_r action => ::first 

do_simple_roll received: ({}, 4, "d", 6)
        Rolled : 3
        Rolled : 2
        Rolled : 5
        Rolled : 6
do_simple_roll returning: { die_type => "1d6", rolls => [3, 2, 5, 6] }
modifier_r received: ({}, { die_type => "1d6", rolls => [3, 2, 5, 6] }
+, "r", 1)
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There are no rules, there are no thumbs..
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Re^3: First steps with Marpa::R2 and BNF
by GrandFather (Saint) on Jan 16, 2021 at 03:44 UTC

Perhaps a full example will help:

use strict;
use warnings;
use Marpa::R2;

my $dsl = <<'END_OF_DSL';
lexeme default = latm => 1
 
Dice_Expression ::= Simple_Dice action => ::first
Simple_Dice ::= Rolls ('d') Sides action => do_simple_roll

Rolls ~ digits
Sides ~ digits
digits ~ [\d]+

:discard ~ whiteSpace
whiteSpace ~ [\s]+
END_OF_DSL
 
 
package Actions;

sub do_simple_roll {
    my (undef, $rolls, $sides) = @_;
    my @res = map {1 + int(rand($sides))} 1 .. $rolls;
    
    return \@res;
}

package main;

my $grammar = Marpa::R2::Scanless::G->new({ source => \$dsl});
my $input = '2d6';
my $rolls = $grammar->parse(\$input, 'Actions');
    
print "Result: @$$rolls\n";
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Note the :default ::= action => ::first. Actually we can omit the default line altogether and then change the Dice_Expression line to:

Dice_Expression ::= Simple_Dice action => ::first
[download]

with the same result. The action determines what is passed up to the next level. If there is no action and no explicit default action undef is passed back up (although that isn't what the documentation seems to say). The default action is used where there is no explicit action. Perhaps the key thing to understand is that the called sub gets a parameter in its argument list for each RHS primary (see RHS alternatives) unless the primary is hidden. RHS values are either constant strings or the LHS for a G1 rule, or a L0 symbol.

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