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