Main.elm

type Tree
    = Node Tree Int Tree
    | Empty


empty : Int -> Tree
empty v =
    Node Empty v Empty


sampleTree =
    Node
        (Node (empty 1) 3 (Node (empty 4) 6 (empty 7)))
        8
        (Node Empty 10 (Node (empty 13) 14 Empty))


treeView : Tree -> Html Msg
treeView tree =
    case tree of
        Node left v right ->
            li []
                [ a [ href "#" ]
                    [ text <| String.fromInt v ]
                , ul []
                    [ treeView left
                    , treeView right
                    ]
                ]

        Empty ->
            li []
                [ a [ href "#" ] [ text "" ]
                ]


view : Model -> Html Msg
view model =
    div [ class "container" ]
        [ div [ class "tree" ]
            [ ul []
                [ treeView sampleTree
                ]
            ]
        ]

Tests.elm

treeViewTest : Test
treeViewTest =
    describe "treeView"
        [ describe "Emptyのとき"
            [ test "Emptyのときは、aタグの中が空文字である。" <|
                \() ->
                    Empty
                        |> treeView
                        |> Query.fromHtml
                        |> Query.find [ Selector.tag "a" ]
                        |> Query.has [ Selector.text "" ]
            ]
        , describe "子を持たないノードのとき" <|
            let
                tree =
                    Node Empty 1 Empty
            in
            [ test "根の数は一つである。" <|
                \() ->
                    tree
                        |> treeView
                        |> Query.fromHtml
                        |> Query.findAll [ Selector.tag "a" ]
                        |> Query.first
                        |> Query.has [ Selector.text "1" ]
            , test "左右の子は値を持たない。" <|
                \() ->
                    tree
                        |> treeView
                        |> Query.fromHtml
                        |> Query.find [ Selector.tag "ul" ]
                        |> Query.findAll [ Selector.tag "a", Selector.text "" ]
                        |> Query.count (Expect.equal 2)
            ]
        , describe "左に子を持つノードのとき" <|
            let
                tree =
                    Node (empty 1) 2 Empty
            in
            [ test "根の数は一つである。" <|
                \() ->
                    tree
                        |> treeView
                        |> Query.fromHtml
                        |> Query.findAll [ Selector.tag "a" ]
                        |> Query.first
                        |> Query.has [ Selector.text "2" ]
            , test "左の子は値を持つ。" <|
                \() ->
                    tree
                        |> treeView
                        |> Query.fromHtml
                        |> Query.findAll [ Selector.tag "ul" ]
                        |> Query.first
                        |> Query.findAll [ Selector.tag "li" ]
                        |> Query.first
                        |> Query.has [ Selector.tag "a", Selector.text "1" ]
            ]
        ]