[SOLVED] How to break outer cycle in Ruby?

How to break outer cycle in Ruby?

In Perl, there is an ability to break an outer cycle like this:

AAA: for my $stuff (@otherstuff) {
         for my $foo (@bar) {
             last AAA if (somethingbad());
         }
      }

(syntax may be wrong), which uses a loop label to break the outer loop from inside the inner loop. Is there anything similar in Ruby?

Solution

What you want is non-local control-flow, which Ruby has several options for doing:

Continuations

Pros:

Continuations are the standard mechanism for non-local control-flow. In fact, you can build any non-local control-flow (subroutines, procedures, functions, methods, coroutines, state machines, generators, conditions, exceptions) on top of them: they are pretty much the nicer twin of GOTO.

Cons:

Continuations are not a mandatory part of the Ruby Language Specification, which means that some implementations (XRuby, JRuby, Ruby.NET, IronRuby) don't implement them. So, you can't rely on them.

Exceptions

Pros:

There is a paper that proves mathematically that Exceptions can be more powerful than Continuations. IOW: they can do everything that continuations can do, and more, so you can use them as a replacement for continuations.
Exceptions are universally available.

Cons:

They are called "exceptions" which makes people think that they are "only for exceptional circumstances". This means three things: somebody reading your code might not understand it, the implementation might not be optimized for it (and, yes, exceptions are godawful slow in almost any Ruby implementation) and worst of all, you will get sick of all those people constantly, mindlessly babbling "exceptions are only for exceptional circumstances", as soon as they glance at your code. (Of course, they won't even try to understand what you are doing.)

throw/catch

This is (roughly) what it would look like:

catch :aaa do
  stuff.each do |otherstuff|
    foo.each do |bar|
      throw :aaa if somethingbad
    end
  end
end

Pros:

The same as exceptions.
In Ruby 1.9, using exceptions for control-flow is actually part of the language specification! Loops, enumerators, iterators and such all use a StopIteration exception for termination.

Cons: