Michael Vanier
I'm sure this has been done a hundred times before, but a simple
occurred to me and I wonder if there's anything like it in the standard
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. So I wrote a more general foldl with an "escape" predicate which terminates
with a function which tells what to return in that case (given an argument of
generalization of foldl just libraries (I couldn't find the evaluation, along the running total 'z'):
foldle :: (b -> Bool) -> (a -> a) -> (a -> b -> a) -> a -> [b] -> a foldle _ _ _ z [] = z foldle p h f z (x:xs) = if p x then h z else foldle p h f (f z x) xs
Using this function, "foldl" is:
foldl' = foldle (const False) id
and "any" is just:
any p = foldle p (const True) const False
There have already been better responses / solutions to this, but I just wanted to point out that there was already a form of an "escaping" left fold, namely foldM.
import Data.Maybe ( isJust ) import Control.Monad ( foldM )
any p = not . isJust . foldM (\_ x -> if p x then Nothing else Just ()) ()
Of course the logic is a little confusing to read :)