Chain together many computations that may fail. It is helpful to see its definition:

andThen : Maybe a -> (a -> Maybe b) -> Maybe b
andThen maybe callback =
    case maybe of
        Just value ->
            callback value


        Nothing ->
            Nothing

This means we only continue with the callback if things are going well. For example, say you need to use (head : List Int -> Maybe Int) to get the first month from a List and then make sure it is between 1 and 12:

toValidMonth : Int -> Maybe Int
toValidMonth month =
    if month >= 1 && month <= 12 then
        Just month


    else
        Nothing


getFirstMonth : List Int -> Maybe Int
getFirstMonth months =
    head months `andThen` toValidMonth

If head fails and results in Nothing (because the List was empty), this entire chain of operations will short-circuit and result in Nothing. If toValidMonth results in Nothing, again the chain of computations will result in Nothing.

Pick the first Maybe that actually has a value. Useful when you want to try a couple different things, but there is no default value.

oneOf [ Nothing, Just 42, Just 71 ] == Just 42


oneOf [ Nothing, Nothing, Just 71 ] == Just 71


oneOf [ Nothing, Nothing, Nothing ] == Nothing