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

findIndexM = (liftM (findIndex id) .) . mapM On 26 Nov 2010, at 22:46, José Romildo Malaquias wrote:

Hello.

I need a function findIndexM, similar to findIndex from the standard module Data.List, but which works with a monadic predicate to test list elements.

findIndex :: (a -> Bool) -> [a] -> Maybe Int

findIndexM :: (Monad m, Num a) => (t -> m Bool) -> [t] -> m (Maybe a)

findIndexM p xs = go 0 xs where go _ [] = return Nothing go n (x:xs) = do res <- p x if res then return (Just n) else go (n+1) xs

How can this function be rewritten using combinators?

Romildo _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe