Apply (a -> Maybe b) function to list returning Nothing if any application results in Nothing, else Just (List b)

maybeMap.elm

import Html exposing (text)

data1 = [ 1, 2, 3, 11 ]

fn1 x = if x < 12 then Just (x + 100) else Nothing

maybeMap : (a -> Maybe b) -> List a -> (Maybe (List b))
maybeMap fn list =
  case list of
    [] -> Just []
    (h :: t) ->
      case fn h of
        Nothing -> Nothing
        Just hb ->
          case (maybeMap fn t) of
            Nothing -> Nothing
            Just tb -> Just (hb :: tb)

main =
  text <| toString <| maybeMap fn1 [10, 11, 2]