[Haskell-cafe] Noob error: Type b -> c b Does not match IO a
p.turner at computer.org
Sun Jun 26 00:06:00 EDT 2005
On 2005 June 25 Saturday 17:49, kynn at panix.com wrote:
> 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?
Actually, foldr is applicable to short-circuiting.
foldr (&&) True
works fine for detecting whether all elements of a list are True, and does not
evaluate the list beyond the first occurrence of False. Similarly, if `m` is
defined as a short-circuiting multiplication operator, i.e.
m x y = if x==0 then 0 else x*y
foldr m 1
does not evaluate the list beyond the first occurrence of 0. Unfortunately,
`m` does not work as well with foldr as &&. The function (foldr m 1) creates
a stack containing one (*) operation for every list element to be multiplied,
which can cause undesirable memory usage.
It's still possible to use fold and get short circuiting with good memory
upTo pred = foldr (\a -> \xs -> if pred a then [a] else a:xs) 
prodList = foldl' (*) 1 . upTo (== 0)
It might be considered cheating, but AFAICT the test for ==0 needs to be
separated from the multiplication proper.
Note, foldl' is a more strict, more useful variant of foldl, located in
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