[Haskell-cafe] Re: version of findIndex that works with a monadic predicate

On Sat, 2010-11-27 at 02:23 +0300, Miguel Mitrofanov wrote:

findIndexM = (liftM (findIndex id) .) . mapM

Not quite. Compare: findIndexM (\x -> print x >> return True) [1,2,3] or findIndexM (\x -> if x == 2 then Nothing else Just True) [1,2,3] Possibly better: findIndexM p = foldr (\(n, y) x -> p y >>= \b -> if b then return $! Just $! n else x) (return $! Nothing) . zip (map fromIntegral [1..])) Regards