That's cool -- good point. takeWhile is also trivially defined in terms of foldr:
takeWhile p = foldr (\x r -> if p x then x:r else []) []
Can you do dropWhile in terms of foldr? I don't see how. Mike Stefan O'Rear wrote:
On Wed, Jul 04, 2007 at 04:20:20PM -0700, Michael Vanier wrote:
I'm sure this has been done a hundred times before, but a simple generalization of foldl just occurred to me and I wonder if there's anything like it in the standard libraries (I couldn't find anything). Basically, I was trying to define the "any" function in terms of a fold, and my first try was this:
any :: (a -> Bool) -> [a] -> Bool any p = foldl (\b x -> b || p x) False This is inefficient, because if (p x) is ever True the rest of the list is scanned unnecessarily.
Rather than create a new escape fold, it's much easier, simpler, and faster just to use a right fold:
any p = foldr (\x b -> p x || b) False
That will short-ciruit well by laziness, and is made tailrecursive by same.
Stefan