Re: [Haskell-cafe] foldlWhile
(opss just noticed I did a reply-to) > > 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 Not really. > returns the first 'a' that does not satisfy 'p'. This should be an > equivalent version: Yeap, just like his, foldlWhile :: (a -> b -> a) -> (a -> Bool) -> a -> [b] -> a foldlWhilefp abs = (_, False) -> a It tests the accumulator with p, and returns it on "false". J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldlWhile
On Sat, Nov 20, 2004 at 03:48:23PM +, Jorge Adriano Aires wrote: > > On Sat, Nov 20, 2004 at 12:47:58PM +0300, Serge D. Mechveliani wrote: > > > foldlWhile :: (a -> b -> a) -> (a -> Bool) -> a -> [b] -> a > > > foldlWhilefp abs = > > > 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 signature.asc Description: Digital signature ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldlWhile
On Sat, Nov 20, 2004 at 12:47:58PM +0300, Serge D. Mechveliani wrote: > Is such a function familia to the Haskell users? > > foldlWhile :: (a -> b -> a) -> (a -> Bool) -> a -> [b] -> a > foldlWhilefp abs = > case > (bs, p a) > of > ([],_) -> a > (_, False) -> a > (b:bs', _) -> foldlWhile f p (f a b) bs' > > foldl does not seem to cover this. Why not just foldlWhile f p a bs = takeWhile p $ foldl f a bs ? > More `generic' variant: > > foldlWhileJust :: (a -> b -> Maybe a) -> a -> [b] -> a > foldlWhileJustf abs = case bs of > >[]-> a >b:bs' -> case f a b of Just a' -> foldlWhileJust f a' bs' > _ -> a I don't know a short way to rewrite this one yet. Peace, Dylan signature.asc Description: Digital signature ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] foldlWhile
Is such a function familia to the Haskell users? foldlWhile :: (a -> b -> a) -> (a -> Bool) -> a -> [b] -> a foldlWhilefp abs = case (bs, p a) of ([],_) -> a (_, False) -> a (b:bs', _) -> foldlWhile f p (f a b) bs' foldl does not seem to cover this. Example. Sum the list while the sum is less than bound: boundSum :: Integer -> [Integer] -> Integer boundSumb = foldlWhile (+) (< b) 0 Example: boundSum 100 $ map (^2) [1 ..] Is this reasonable? More `generic' variant: foldlWhileJust :: (a -> b -> Maybe a) -> a -> [b] -> a foldlWhileJustf abs = case bs of []-> a b:bs' -> case f a b of Just a' -> foldlWhileJust f a' bs' _ -> a Please, copy the replies to [EMAIL PROTECTED] - Serge Mechveliani [EMAIL PROTECTED] ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
