{-| Returns a list of repeated applications of `f`.

If `f` returns `Nothing` the iteration will stop. If it returns `Just y` then
`y` will be added to the list and the iteration will continue with `f y`.

    nextYear : Int -> Maybe Int
    nextYear year =
      if year >= 2030 then
        Nothing
      else
        Just (year + 1)

    -- Will evaluate to [2010, 2011, ..., 2030]
    iterate nextYear 2010
-}
iterate : (a -> Maybe a) -> a -> List a
iterate f x =
  case f x of
    Just x' -> x :: iterate f x'
    Nothing -> [x]