Adding 'goto' to your Perl 6 program

masak on 2010-01-08T16:22:12

Today I tested something that jnthn++ and I had discussed during a walk in the non-tourist parts of Riga after Baltic Perl Workshop.

$ cat test-goto
Q:PIR {
  line_10:
};

say "OH HAI!";

Q:PIR {
  goto line_10
}

$ perl6 test-goto
OH HAI!
OH HAI!
OH HAI!
OH HAI!
[...]

Oh. My. Wow.

The 1980's called; they want their infinitely looping toy BASIC idiom back.

I half-expected that not to work, but I'm glad it does. I can even imagine it being of actual, code-simplifying use in some applications. The reports of the harmfulness of GOTO have been greatly exaggerated, if you ask me. Like everything else, the goto keyword shouldn't be overused, but a well-placed goto LABEL can actually improve readability. Often these masquerade as next LABEL or last LABEL or redo LABEL in Perl loops. But those are gotos with a nicer accent, a briefcase, and a better salary.

Unfortunately, the trick doesn't take us very far. Since we're using PIR gotos, we can only jump around within the same sub. Not just the same Perl 6 sub, that is, but the same PIR sub. Since every block in Perl 6 corresponds to a sub in PIR, we can't jump outside of the block.

$ cat test-goto-loop 
loop {
    say "OH HAI!";
    Q:PIR {
      goto line_10
    }
}

Q:PIR {
  line_10:
};

$ perl6 test-goto-loop 
e_pbc_emit: no label offset defined for 'line_10'
in Main (file , line )

Well, that certainly makes it less useful. Shame.

Now, how about them PIR-based continuations...? ☺

PIR continuations

tene on 2010-01-08T18:37:39

Re:PIR continuations

masak on 2010-01-08T23:20:07

I dunno. I just want to do indecent things with them. Nothing specific.

Actually, the one specific use that comes to mind is for GGE, my PGE port. PGE uses coroutines and continuations quite liberally, and so far in GGE, I've emulated continuations by building my own call stack and lexical pads. Maybe I could get away with making a few PIR detours for continuations instead.