http://www.perlmonks.org?node_id=425815


in reply to Re: RFC: Implicit Parallelization Pragma
in thread RFC: Implicit Parallelization Pragma

Indeed. And in Perl, this is very, very hard. How hard? Well, your own example fails the test. Even the map {length} @array may cause a side-effect.

Here's another example:

use warnings; use strict; { package Tmp; use overload '""' => \&str, fallback => 1; sub c { bless [$_[1]], $_[0] } sub str { print "Hello, $_[0][0]\n"; "." x rand 10 } } my @strs = map Tmp->c($_), qw[foo bar baz]; my @str_lengths = map { length } @strs; print "$_\n" for @str_lengths;

As you can see, this time each call to length prints something to STDOUT and calls rand, which modifies global state. This is another reason why perl can't optimize such a loop in the general case: it's very hard to prove that @strs isn't tied or contains objects with overloaded stringification.