Michael Limberger

Perl

Goto loops

A label inside else, then jump

2024-06-28

This note looks at goto-style loops in Perl. They are unfashionable, and sometimes they are still the clearest way to say what you mean.

#!/usr/bin/env perl
use feature qw|say|;

my $n = 7;

if ($n == 10)
{
    say qq|I see a ${n}!|;
}
else
{ R:
    say qq(Hmmm... looking for 10... \$n = ${\$n++});
    sleep 1;
    $n > 10 ? say qq(Yay! It's a ${\--$n}!) : goto R;
}

It counts from 7 toward 10, then celebrates. The overshoot to 11 is on purpose: the exit test is $n > 10, then a pre-decrement prints 10.

The landing pad

R: is a label. Labels do not create scope. Perl will jump to one it can see, even inside else.

Three flavours

`goto LABEL`	Classic jump. What this piece uses.
`goto &NAME`	Replace the current sub. A real tail call.
`goto EXPR`	Computed destination. Do not.

Never use goto is too strong. Tail calls and generated state machines have a case. Building a loop inside else is showing off. Save it for golf.