Well, the case of the built-in numeric types is somewhat different,
but most functions automatically short circuit, since Haskell uses
lazy evaluation.
For instance, it's perfectly okay to define
myAnd = foldr (&&) True
Note that this terminates on infinite lists which contain False as a value:
myAnd (False : repeat True)
will evaluate to False pretty much immediately.
- Cale
On 25/06/05, kynn@panix.com
Some simplifications might help you here...
prodList [] = 1 prodList (0:xs) = 0 prodList (x:xs) = x * prodList xs
Simplified: prodList xs = foldl (*) 1 xs
But my original at least made some provision for short circuiting the whole operation if the list contained a 0. As far as I can figure, fold, map, etc., are not suitable for any situation in which short-circuiting would be desirable (e.g. and, or, etc.). Am I wrong?
(I realize that, given that the function product is already available, there is no point in defining my function prodList above, but my question is a more general one, having to do with how best to implement short-circuiting.)
Thanks!
kj
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