From: Paul Brannan <pbrannan@...>
Date: 2007-12-29T03:37:16+09:00
Subject: Re: def if ; end - basically impossible?

On Sat, Dec 29, 2007 at 01:21:04AM +0900, Giles Bowkett wrote:
> I'm writing some code which needs to do something unusual when it
> encounters an if, an else, or an end.
> 
> >> def if
> >>   "asdf"
> >>   end
> => nil
> >> if
> ?>     true
> >>   end
> => nil
> 
> The ?> line comes up because if appears to not be a method but an
> operator or something similar, so my override does nada.
> 
> Is there any way to do this? It looks as if I have to either rethink
> my strategy or write some kind of parser.

It works if you call #if on an object.  I do something like this in
ruby-libjit:

  class Function
    def if(condition, &block)
      # append code to:
      #   - if the condition is true, execute the code appended by the
      #     block
      #   - if the condition is false, branch to the end label
      block.call
      return If.new(self)
    end

    class If
      def initialize(function)
        @function = function
      end

      def end
        # append the end label here
      end
    end
  end

so I can write something like:

  Function.compile do |f|
    f.if(<condition>) {
      # true case
    } .else {
      # false case
    } .end

  end

(I've omitted the code for #else for simplicity, because it really makes
the code complicated)

Paul