perl - 将哪些参数传递给 Marpa::R2 操作？

Question

在 cpan Marpa::R2的页面，我了解 BNF(Backus-Naur 形式)，但我对 Action 回调非常迷茫。

在下面这个例子中，我了解到左右两个成员被传递给 do_multiply .对于那件事我没有任何疑问。问题是我找不到关于这些论点是什么的任何文档？

my $dsl = <<'END_OF_DSL';
:default ::= action => [name,values]
lexeme default = latm => 1    
Calculator ::= Expression action => ::first
...
    Term '*' Factor action => do_multiply
...
END_OF_DSL

my $grammar = Marpa::R2::Scanless::G->new( { source => \$dsl } );

sub do_multiply    { $_[1] * $_[2] }   

什么是 $_[0]甚至 $_[3] ?这是在哪里记录的？连上 official marpa website我没有看到任何文档。

在另一个示例中， here chobora 的回答，我们看到 pair指 $_[2]和 $_[3] :

BNF的片段:

Hash  ::= '(' Pairs ')'     action => hash
Pairs ::= Pair+             action => pairs
Pair  ::= '(' Key Value ')' action => pair
Key   ::= String

核心代码:

$recce->read(\$input);
print Dumper $recce->value;

sub hash   { $_[2] }
sub pairs  { shift; +{ map @$_, @_ } }
sub pair   { [ @_[2, 3] ] }               # What is 2 and 3?
sub itself { $_[1] }

Answer 1

Marpe::R2 docs说以下几点:

sub My_Actions::do_add {
    my ( undef, $t1, undef, $t2 ) = @_;
    return $t1 + $t2;
}

The Perl semantic closures are callbacks. They are called as each node in a parse tree is evaluated.

Each Perl semantic closure is called with one or more arguments. The first argument to a value action is always a per-parse-tree object, which the callbacks can use as a scratchpad. In this example, the per-parse-tree object is not used. The remaining arguments will be the values of the node's "children" -- in other words, the values computed for each of its RHS symbols, in order. If the action is for an empty rule, the per-parse-tree object will be its only argument.

Every value action is expected to return a value. With one exception, this value is passed up to a parent node as an argument. The exception is the value for the start rule. The return value for the start rule becomes the parse result.

所以这些函数被传递一个对象作为第一个参数。那是 $_[0] .

对于您的示例，该文本似乎意味着以下参数， $_[1]和 $_[2]只是你用来计算东西的值。

添加一些调试输出给了我以下内容:

use Data::Printer;

sub My_Actions::do_add {
    my ( undef, $t1, undef, $t2 ) = @_;
say 'do_add';
p @_;

    return $t1 + $t2;
}

sub My_Actions::do_multiply {
    my ( undef, $t1, undef, $t2 ) = @_;
say 'do_multiply';
p @_;
   return $t1 * $t2;
}

__END__

do_multiply
[
    [0] {},
    [1] 42,
    [2] "*",
    [3] 1
]
do_add
[
    [0] {},
    [1] 42,
    [2] "+",
    [3] 7
]