parsing recursive, lisp-like JSON in elm

TreeOfInt.elm

-- I feel very uncertain this is the Right Thing.
-- tested against Elm 0.19

import Json.Decode exposing (..)
type TreeOfInt
    = Branch (List TreeOfInt)
    | Leaf Int

-- decodeString treeOfInt "[7, [10]]" => Branch [Leaf 7, Branch [Leaf 10]]
treeOfInt : Decoder TreeOfInt
treeOfInt =
     let
         f : Maybe Int -> Decoder TreeOfInt
         f maybeInt =
             case maybeInt of
                 Just i ->
                     succeed (Leaf i)
                 Nothing ->
                     andThen g (list treeOfInt)
         g : List TreeOfInt -> Decoder TreeOfInt
         g = succeed << Branch
     in
     andThen f (maybe int)