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Ovid (2709)

Ovid
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Stuff with the Perl Foundation. A couple of patches in the Perl core. A few CPAN modules. That about sums it up.

Journal of Ovid (2709)

Wednesday May 21, 2008
03:17 AM

From Ruby To Perl

[ #36480 ]

I've spent a bit of time translating a small Ruby program to Perl. If you've not actually programmed in Ruby before, it's an interesting language and not all idioms translate well to Perl. There are no strictures, so variables pop into existence upon first use. Hope you don't misspell them!

Curiously, though, that seems to eliminate one (minor) nit I've always had: the declaration of recursive anonymous functions. Consider this Perl:

my $fact = sub {
    my $num = shift;
    return $num < 2 ? 1 : $num * $fact->($num - 1);
};

That doesn't actually work because $fact doesn't exist at the time you call it. So you need to do this:

my $fact;
$fact = sub {
    my $num = shift;
    return $num < 2 ? 1 : $num * $fact->($num - 1);
};

Now, depending on whether or not you like the ternary operator, this is straightforward, idiomatic Perl. It's also just a tad ugly, having to declare that variable before you can use it. Here's the Ruby equivalent:

fact = proc { |num|
    return num < 2 ? 1 : num * fact[num - 1]
}

Also, I liked this really nifty local redefining of the << operator (typically this is the Ruby equivalent of 'push'):

def <<(rhs)
  case rhs
    when Array
      fact(*rhs)
    else
      fact(rhs)
  end
end

While without context it doesn't mean much, it does simplify some of the code interacting with the object this is redefined for.

Now I'm tangling with the 'yield' keyword. I know what it does, but translating it idiomatically to Perl is interesting.

Any Ruby equivalent of perldoc -f?

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  • That doesn’t actually work because $fact doesn’t exist at the time you call it.

    So you simply use the Y combinator [perl.org]. :-)

    • I had thought of that when I was writing this, but I confess that I never really understood why people were so hot and bothered about the Y combinator. There's a point when complexity overrides functionality. That being said, it may simply be a case of my being dense, but an awful lot of smart people seem to struggle with this :)

      • Complexity of what? The Y combinator is easy to use: you return the recursive closure from another closure; the first argument of the outer closure can be used in the inner closure to return. That’s it.

        The difficulty is in deriving the exact form of the combinator (which, of course, is a bit of brain-melt, which is why it’s become a sort of sport), not in using it.

  • rubyisms.pm has an idiomatic translation of "yield" (As well as "self" and "super"). As for perldoc -f, ri should do everything you want.
    • I'm working with the 'yield' function from that now. And thanks for the tip about 'ri'. Very useful!

    • Actually, I can't seem to use your 'yield' as a direct translation. I'm missing something fundamental. I put together this in Ruby (very similar to my actual problem):

      class Foo
          attr_reader :val
          def initialize(val)
              @val = val
          end
          def inc(some_val)
              @val = @val + some_val
          end
      end

      def testy(args)
          foo = Foo.new(1)
          _testy(args, foo) {
              yield

  • See, this is exactly why I love Perl, cause there is always a something on CPAN that will make your life easier.

    use Sub::Current;

    print sub {
        my $num = shift;
        return $num < 2 ? 1 : $num * ROUTINE->($num - 1);
    }->(10);

    - Stevan

    • That's great, but solve my 'yield' problem and I'll really love you :)

      • Well, all the "yield" and "block" stuff is just sugar around your basic continuation passing. So here is the translation of what the ruby is actually doing.

        #!/usr/bin/perl

        use strict;
        use warnings;

        {
            package Foo;
            sub new { bless { val => $_[1] } => $_[0] }
            sub inc { $_[0]->{val} += $_[1] }
            sub val { shift->{val} }
        }

        sub testy {
            my ($args, $cont) = @_;
            my $foo = Foo->new(1);
            _testy($args, $foo, sub { $

      • I took the dog for a walk and thought about this a bit and realized where my scoping issue was. Here is a sugared version, probably could be a little cleaner still, maybe even with some subroutine attributes (sub foo : continuation { ... } or something).

        #!/usr/bin/perl

        use strict;
        use warnings;
        use Scalar::Util 'blessed';

        {
            package Foo;
            sub new { bless { val => $_[1] } => $_[0] }
            sub inc { $_[0]->{val} += $_[1] }
            sub val { shift->{val} }
        }

        our

        • Damn. I am doing the bit with passing around the sub, but it really does read like what you've done is much cleaner and actually makes it read like Ruby.

          • Actually took a look into rubyisms, I think that is just another case of Cozens "egoware" (only one version ever released and not meant to actually be used, only for people to look at and go "wow"). All the DB:: fiddling means it is bound to be very fragile with things like eval, etc.

            RE: reading like Ruby

            I actually find the desugared perl one to be the most readable since it doesn't obscure what is actually being done.

            - Stevan

        • Ruby's use of the 'yield' keyword caught me off guard. I suppose I'm used to how python uses it, as part of generators (rather than as simply a sub invocation). Well... here's how you might do something similar with Coro::Generator anyway :)

          use strict;
          use Coro::Generator;

          {
            package Foo;
            sub new { bless { val => $_[1] } => $_[0] }
            sub inc { $_[0]->{val} += $_[1] }
            sub val { shift->{val} }
          }

          my $inner_testy = generator {
            my $foo = pop;
            foreach my $item (@_) {