in reply to Re^2: Accessing lexicals in other scopes dynamically by name
in thread Accessing lexicals in other scopes dynamically by name

Nothing's captured. $a, $b, $c are still in scope, and we've already established that eval should and can access lexicals in scope.

Place all by the calls to f() in curlies or in a module and you'll see it doesn't capture. 

use strict;

{

my $a = 1;
my $b = 2;
my $c = 3;

sub f {
  my $var = shift;
  print $var, "=", eval "\$$var", "\n";
};

}

&f("a");
&f("b");
&f("c");

[download]
$ perl -w a.pl
Variable "$a" is not available at (eval 1) line 2.
Use of uninitialized value in print at a.pl line 11.
a=
Variable "$b" is not available at (eval 2) line 2.
Use of uninitialized value in print at a.pl line 11.
b=
Variable "$c" is not available at (eval 3) line 2.
Use of uninitialized value in print at a.pl line 11.
c=

[download]

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Re^4: Accessing lexicals in other scopes dynamically by name
by morgon (Priest) on Jul 30, 2010 at 17:48 UTC
    ikegami, you are right.

    Does anyone have any idea why Perl does it like that?

    I mean to implement it like this must be more difficult than simply to keep a pointer to the whole enviroment for every closure.

    What is the motivation?

[reply]
      I think it's an optimisation. Capturing all lexicals would cause a closure to take longer to create, use more memory and prevent another optimisation that reuses lexicals from previous calls to the function.
[reply]
        Hmm.

        I have never really looked at the Perl-internals, but I can vagly remember having seen an implementation of Scheme in Scheme.

        If I remember correctly there was a data-structure containing all the variable-bindings for any lexical scope and every closure would simply keep a reference to this.

        So in this approach there is not a "capturing of all lexicals" but simply remembering one data-structure regardless of how many variables were defined it.

        Perl evidently does this differently and I even think that the inaccessabilty of certain lexial variables of which Perl could not see that the are going to be used in a closure is a weakness.

        I will try this in Rakudo.

[reply]

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