Tree.elm

module Tree exposing (..)

import Html exposing (text)
import Json.Decode exposing (..)
import Json.Decode.Extra exposing (lazy)


type BST
    = Branch BST Int BST
    | Leaf


bst : Decoder BST
bst =
    oneOf
        [ null Leaf
        , object3
            Branch
            ("left" := (lazy (\_ -> bst)))
            ("value" := int)
            ("right" := (lazy (\_ -> bst)))
        ]


data =
    """{
    "value": 2,
    "left": {
        "value": 1,
        "left": null,
        "right": null
    },
    "right": {
        "value": 3,
        "left": null,
        "right": null
    }
}"""


goal : BST
goal =
    Branch
        (Branch Leaf 1 Leaf)
        2
        (Branch Leaf 3 Leaf)


main =
    data
        |> decodeString bst
        |> toString
        |> text