Dear Monks and Nuns,

lately I came across an issue with dereferencing array refs. It looks like there is some hidden "lazy deref" when an array ref is used in a foreach loop, compared to the usage as a sub argument. Consider these two subs that do nothing but die. The main difference is the array dereference as an argument to foreach or map. 

use experimental 'signatures';

sub map_die ($ar) {
    eval {map {die} @$ar};
}

sub for_die ($ar) {
    eval {
        for (@$ar) {
            die;
        }
    }
}

[download]
Benchmarking is amazing: 
use Benchmark 'cmpthese';

my @arr = ((0) x 1e6);

cmpthese(0, {
        map => sub {map_die(\@arr)},
        for => sub {for_die(\@arr)},
    });
__DATA__
         Rate     map     for
map    1257/s      --   -100%
for 1664823/s 132352%      --

[download]

Then I remembered the "lazy generators" from List::Gen and gave it a try. There is some progress, but it cannot come up to foreach. 

use List::Gen 'array';

sub gen_die ($ar) {
    eval {
        &array($ar)->map(sub {die});
    }
}

cmpthese(0, {
        map => sub {map_die(\@arr)},
        for => sub {for_die(\@arr)},
        gen => sub {gen_die(\@arr)},
    });
__DATA__
         Rate     map     gen     for
map    1316/s      --    -93%   -100%
gen   18831/s   1330%      --    -99%
for 1662271/s 126174%   8727%      --

[download]

Wouldn't it be nice to have some kind of "explicit lazy dereferencing" in Perl?

Update May 10, 2024: Added the signatures feature as suggested by Danny in Re: The Virtue of Laziness.

Greetings,
🐻

$gryYup$d0ylprbpriprrYpkJl2xyl~rzg??P~5lp2hyl0p$

In reply to The Virtue of Laziness by jo37

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