A neat little recursive data structure I'm using in an Elm project.

OptionTree.elm

module OptionTree exposing (..)



type Node meta a
  = Tree meta (List (Node meta a))
  | OneOf (List a)
  | ManyOf (List a)


select : Node meta item -> item -> List item -> List item 
select node item list =
  case node of
    Tree _ leaves ->
      List.foldl (\leaf -> select leaf item) list leaves

    OneOf items ->
      if List.member item items then
        item :: (List.filter (not << (flip List.member) items) list)
      else
        list

    ManyOf items ->
      if List.member item items then
        item :: list
      else
        list