From: Daniel Lewis <danieljohnlewis@...>
Date: 2005-10-01T21:59:53+09:00
Subject: Re: Lazy evaluaton (was Re: In your opinion....)

On 01/10/05, Robert Klemme <bob.news@gmx.net> wrote:
> Devin Mullins <twifkak@comcast.net> wrote:
> > 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
>
> Delegate comes to mind.  Not really shorter...
>
> require 'delegate'
> class Lazy < Delegator
>   alias :init :initialize
>
>   def initialize(&b)
>     super(nil)
>     @bl = b
>   end
>
>   def __getobj__
>     if @bl
>       @obj = @bl.call()
>       init(@obj)
>       @bl = nil
>     end
>     @obj
>   end
> end
> def lazy(&b)Lazy.new(&b)end
>
> Only advantage seems that one can reuse lib code for delegation and
> initialization.
>
> Kind regards
>
>     robert
>
>
>

oooh my thread has split :)

I'm going to be noting all this down, Lazy Evaluation is a subject in
Functional Programming and AI.

Thanks,

Daniel.