General approach to modifying an array while looping through it

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Is it safe in Perl6 (as opposed to perl5 or other languages) to loop through an array while modifying it? For instance, if I have an array of websites to download, and I add failed downloads to the end of the array to re-download, will perl6 behave as expected? (I have about 50k links to download and trying to test all out would be time consuming.)

If not safe, what is a general approach? I have been thinking of storing links of interrupted downloads in another array, and loop through that array after original array was done. However, this is like a fox chasing its tail because I have to store failed downloads in another array (or overwrite the original array).

raku arrays loops
1 Answers

It is definitely safe in a single-threaded environment:

my @a = ^5;
for @a { 
    @a.push: $_ + 10 if $_ < 30
}
say @a

[1 2 3 4 11 12 13 14 21 22 23 24 31 32 33 34]

In a multi-threaded environment (which is what is better be used in your task) nothing can be taken for granted. Therefore appending of a new element to the array is better be wrapped into a Lock:

my @a = ^5;
my Lock $l .= new;
for @a {
    start {
        ... # Do your work here 
        $l.protect: {
            @a.push: $_ with $site
        }
    }
}
say @a

Note that the last sample won't work as expected because all started threads must be awaited somewhere within the loop. Consider it just as a basic demo.

Yet, locking is usually be avoided whenever possible. A better solution would use Channel and react/whenever blocks.

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