afraid not
the given example is too strict, the requirement is to generate the matched portion lazilly, and return the tail (unconsumed portion).
ah yes, without optimisations, Prelude.span builds up stack, while the continuation-based alternative i mentioned is too strict for some uses.
In principle the function should be capable of being written to run in constant space which the given example dose not.
return (repeat 'a') >>= \ x -> print $ span (const True) x
how about the old spec, then? span p l = (takeWhile p l,dropWhile p l) since takeWhile takes forever, here, it isn't even inefficient!-) claus
with hugs you will get a stack error, in ghc it executes in constant space, i.e. indefinitely. In essenece the above example does exactly the same as my ealier code.
this thread might be relevant:
http://www.haskell.org/pipermail/hugs-bugs/2007-June/001815.html http://www.haskell.org/pipermail/hugs-bugs/2007-June/001816.html http://www.haskell.org/pipermail/hugs-bugs/2007-June/001817.html