Generate Trees in Elm

Trees.elm

import Html exposing (Html, text, div, button)
import Html.App exposing (beginnerProgram)
import Html.Events exposing (onClick)
import Random exposing (Generator, Seed, step, initialSeed, andThen, map, map2, int)


main =
  beginnerProgram {
      model = init 0
    , view = view
    , update = update
  }


type Tree =
    Leaf Int
  | Node Tree Tree


tree : Generator Tree
tree =
  let
    createNode left right =
      Node left right
    
    selectTreeGenerator : Int -> Generator Tree
    selectTreeGenerator n =
      case n of
        0 -> map2 createNode tree tree
        
        _ -> map (\n -> Leaf n) (int 0 10)
  in
    (int 0 2) `andThen` selectTreeGenerator


type alias Model = { tree : Tree, seed : Seed }


init : Int -> Model
init n =
  let
    seed = initialSeed n
    
    (t, seed') = step tree seed
  in
    { tree = t, seed = seed' }


type Message =
    DoNothing
  | GenerateTree


update : Message -> Model -> Model
update message model =
  case message of 
    DoNothing -> model
    
    GenerateTree ->
      let
        (t, seed') = step tree model.seed
      in
        { model | tree = t, seed = seed' }

view : Model -> Html Message
view model =
  div [] [
    button [onClick GenerateTree ] [ text "go" ]
  , text (toString model.tree)
  ]