02_regex_backtracking_hang.p6

#!/usr/bin/env perl6

# Golfed-down demo for regex backtracking problem affecting http://rosettacode.org/wiki/Amb#Perl_6
# and previously explored in https://gist.github.com/smls/85b78904e96eb589ea1e

# This works:

{
    '' ~~ m/
        :my ($a, $b);
        [{ $a = 1 } || { $a = 0 }]
        [{ $b = 0 } || { $b = 1 }]
        <?{ all($a, $b) == 1 }>
        { say "$a $b" }
    /;
}

say "---";

# But this hangs indefinitely:

{
    '' ~~ m/
        :my ($a, $b);
        <{ '[{ $a = 1 } || { $a = 0 }]' }>
        <{ '[{ $b = 0 } || { $b = 1 }]' }>
        <?{ all($a, $b) == 1 }>
        { say "$a $b" }
    /;
}

# The only difference between the two is using:
#     FOO           -- literal sub-regex 
# vs
#     <{ 'FOO' }>   -- eval'ed sub-regex