On Sat, Nov 20, 2004 at 03:48:23PM +0000, Jorge Adriano Aires wrote:

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On Sat, Nov 20, 2004 at 12:47:58PM +0300, Serge D. Mechveliani wrote:

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foldlWhile :: (a -> b -> a) -> (a -> Bool) -> a -> [b] -> a foldlWhile f p a bs = case (bs, p a) of ([], _ ) -> a (_, False) -> a (b:bs', _ ) -> foldlWhile f p (f a b) bs'

Why not just foldlWhile f p a bs = takeWhile p $ foldl f a bs

Quite different. The former stops a foldl when the accumulating parameter no longer satisfies p, the later assumes the accumulating parameter of the foldl is a list, and takes the portion of the list that does satisfy p.

Yes, this was a mistake.

The following is closer to the original, but doesn't work when the whole list is folded (i.e., p always satisfied): foldlWhile f p a = head . dropWhile p . scanl f a

Serge's version returns the last 'a' that satisfies 'p', while yours returns the first 'a' that does not satisfy 'p'. This should be an equivalent version: foldlWhile f p a = tail . takeWhile p . scanl f a But what about the version with Maybe? There ought to be a concise way to write that too. Peace, Dylan