Standard Haskell function :: (a -> Maybe b) -> [a] -> Maybe b

Question

There is a standard tryPick function if F# that returns the first (from left-to-right if any at all) successful application of a function on an element of a list. I am hopping there is a standard function like that in Haskell. I tried Hoogle and didn't find anything.

I am new to Haskell and I am not sure what the right way of doing it is. Would you do it like this:

tryPick:: (a -> Maybe b) -> [a] -> Maybe b
tryPick try xs = case Maybe.mapMaybe try xs of
    [] -> Nothing
    (x:_) -> Just x

?

Answer 1

You want:

tryPick :: (a -> Maybe b) -> [a] -> Maybe b
tryPick f as = msum (map f as)

I'll explain how this works.

map f as produces a list of possible Maybe actions to try:

map f as :: [Maybe b]

msum tries them sequentially until one succeeds (returning the value as a Just) or they all fail (returning a Nothing). For example:

> msum [Nothing, Just 2, Just 3, Nothing]
Just 2
> msum [Nothing, Nothing]
Nothing

Note that msum's type is more general, so we can generalize the signature to:

tryPick :: (MonadPlus m) => (a -> m b) -> [a] -> m b

This will now work for any MonadPlus. Have fun discovering what it does for other MonadPlus types.