Re: [Haskell-cafe] ANNOUNCE: The Monad.Reader - Issue 6

dw :: (a -> Bool) -> [a] -> [a] dw p = reverse . fst . foldl comb ([],False) where comb (xs,done) x | done = (x:xs, True) | p x = (xs, False) | otherwise = (x:xs, True)

Which is the simplest working algorithm I could come up with; sadly it breaks the lazinesss constraint.

Speaking of the laziness constraint, the simplest solution to the strictness of dwBackwards (solution 1) would be to use irrefutable pattern matching in the combine function: dwBackwards predicate = fst . foldr combine ([],[]) where -- Note the tilde in the next line... combine x ~(ys,xs) | predicate x = (ys, x:xs) | otherwise = (x:xs, x:xs)