From: Luke Graham <spoooq@...>
Date: 2005-03-11T22:58:44+09:00
Subject: Re: Is iterating in lock-step possible?

Just found this, it seems to fit the bill nicely and its in the std
lib of all places :P

http://www.ruby-doc.org/stdlib/libdoc/generator/rdoc/classes/Generator.html


On Fri, 11 Mar 2005 23:44:39 +1000, Luke Graham <spoooq@gmail.com> wrote:
> Gah, always miss something when I post code...
> 
> Replace the first two lines of nextco with
>         def nextco
>                 each_with_index { |e,i|
> 
> 
> On Fri, 11 Mar 2005 23:41:46 +1000, Luke Graham <spoooq@gmail.com> wrote:
> > On Fri, 11 Mar 2005 15:57:08 +0900, Roshan James <roshanj@microsoft.com> wrote:
> >
> > > I was trying to write code where I can have two or more computations
> > > which generate values that I need to compare - with the ruby syntax
> > > tying iterator usage to the code blocks by syntax, there is no way I can
> > > do this. This is especially applicable when the iterators potentially
> > > return an infinite stream of values. (Except for using contiuations...
> > > Which is not really so much of a solution, because I might as well not
> > > have used an iterator in a first place if I could create good
> > > coroutines)
> >
> > Continuations are the right answer in this case.
> >
> > module Enumerable
> >         attr_accessor :co, :st, :res
> >
> >         def nextco
> >                 each { |e|
> >                         callcc { |cc|
> >                                 @co = cc
> >                                 @res = e
> >                                 @st.call
> >                         }
> >                 }
> >                 @res = nil
> >                 @st.call
> >         end
> >         def next
> >                 callcc { |cc|
> >                         @st = cc
> >                         @co ? @co.call : nextco
> >                 }
> >                 return @res
> >         end
> > end
> >
> > a = [1,2,3]
> > b = [4,5,6,7,8]
> >
> > while x = a.next and y = b.next
> >         puts x,y
> > end
> >
> > Note there are three possible behaviours around the @res = nil
> > line. One is to return nil after all items have been used up, another
> > is to keep returning the last item by removing that line, and finally
> > a loop can be wrapped around the each, to begin returning from
> > the first item again.
> >
> > Its probably possible to do it in one function, but it makes more
> > sense to me in two.
> >
> > > Yes I understand that I take a hit (wrt perf by choosing ruby), but
> > > that's a BAD argument to favour wrapping state first by a iterators and
> > > then once over by a continuation.
> >
> > As long as its wrapped safely and nicely, what does it matter?
> > This way, you even get your choice of what to do when you
> > run out of items ;)
> >
> > --
> > spooq
> >
> 
> --
> spooq
> 


-- 
spooq