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

[Maciej Piechotka]

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
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