Ben Franksen <[EMAIL PROTECTED]> wrote: > Achim Schneider wrote: > >> ChrisK <[EMAIL PROTECTED]> wrote: >> >>> zeroNothing Nothing = Nothing >>> zeroNothing (Just n) = >>> if n == 0 then Nothing else (Just n) >>> >>> versus >>> >>> zeroNothing Nothing = Nothing >>> zeroNothing x@(Just n) = >>> if n == 0 then Nothing else x >>> >> versus >> >> zeroNothing Nothing = Nothing >> zeroNothing x = >> let (Just n) = x >> in if n == 0 then Nothing else x >> >> so, @ is kind of like a let, just with its arguments flipped. > > However, if x@(Just n) fails to match, the next clause is chosen, > whereas the variable pattern x matches always. Thus, the last > version works only because the other possible case (Nothing) has > already been handled. IOW, in the second version of zeroNothing you > may swap the order of patterns, but not in the third one. > Actually, I considered working it out to
nothingIf :: (a -> Bool) -> Maybe a -> Maybe a nothingIf f m = m >>= (\j -> if f j then Nothing else Just j) nothingIf' :: (a -> Bool) -> Maybe a -> Maybe a nothingIf' f m = m >>= (\j -> if f j then Nothing else m) zeroNothing = nothingIf (== 0) zeroNothing' = nothingIf' (== 0) , but was too lazy. Hopefully only because it completely messes up my point. OTOH, zeroIf :: MonadPlus m => (a -> Bool) -> m a -> m a zeroIf f m = m >>= (\nz -> if f nz then mzero else m) zeroZero :: (MonadPlus m, Num a) => m a -> m a zeroZero = zeroIf (==0) makes it interesting again as you can't construct a Just value with it. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
