From: Devin Mullins <twifkak@...>
Date: 2005-10-01T12:28:04+09:00
Subject: Lazy evaluaton (was Re: In your opinion....)

gabriele renzi wrote:

> Think for example of the IO monad: it allows you to state that you 
> should, say, print "hello" before "world".
> The rest of the program is still happy, functional and lazy, since he 
> does'nt really care about the console output.

The word "lazy", and the impression I was forming about monads from 
reading these posts, inspired this simple doodad. Is this useful? Is it 
already implemented in some other Ruby library out there?

class Lazy
  (instance_methods - %w{__send__ __id__}).each {|meth| undef_method meth }
  def initialize(blk)
    @blk = blk
  end
  def method_missing(*a,&b)
    @inst = @blk.call unless defined? @inst
    @inst.send(*a,&b)
  end
end

def lazy(&blk)
  Lazy.new blk
end

a = lazy { puts "evaled!"; $some_expensive_db_operation or 5 }
puts "Hello!"
p a
p a
__END__

Output:
Hello!
evaled!
5
5

Devin