Andreas Rossberg [mailto:rossberg@ps.uni-sb.de] wrote:
> 
> For real mapping (as opposed to iteration) terminating early makes
> little sense. For iteration and folds it does. It is not too difficult
> to provide variations of iter and folds for your data structures that
> allow early breaks. For example, consider:
>
> BTW, this is one of the few problems where lazy evaluation 
> can play out its strengths: in a lazy language you do not need
> variations of iteration with early termination - you will always
> "terminate early" automatically.

I posted a similar question to comp.lang.functional a few months
ago, and Dan Wang showed me how I could use monads to make a
regular fold do that. 

I actually wanted to thread a state through a fold, but IIRC the 
technique will generalize to any monad (such as the exception
or continuation monad). Google reveals that the thread is at:

http://groups.google.com/groups?threadm=slrn9ig78d.gpl.neelk%40brick.cswv.co
m

--
Neel Krishnaswami
neelk@cswcasa.com
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