Problem 7

Flatten a nested lists into a single list. Because Lists in Elm are homogeneous we need to define what a nested list is.

type NestedList a = Elem a | List [NestedList a]

Example

nl1 =
    SubList
        [ Elem 1
        , SubList
            [ SubList
                [ Elem 2
                , SubList [ Elem 3, Elem 4 ]
                ]
            , Elem 5
            ]
        , Elem 6
        , SubList [ Elem 7, Elem 8, Elem 9 ]
        ]
flatten nl1 == List.range 1 9

Unit Test

import Html
import List


type NestedList a
    = Elem a
    | SubList (List (NestedList a))


flatten : NestedList a -> List a
flatten nl =
    []


main : Html.Html a
main =
    Html.text
        <| case test of
            0 ->
                "Your implementation passed all tests."

            1 ->
                "Your implementation failed one test."

            x ->
                "Your implementation failed " ++ (toString x) ++ " tests."


nl0 =
    SubList [ Elem 1, Elem 2 ]


nl1 =
    SubList
        [ Elem 1
        , SubList
            [ SubList
                [ Elem 2
                , SubList [ Elem 3, Elem 4 ]
                ]
            , Elem 5
            ]
        , Elem 6
        , SubList [ Elem 7, Elem 8, Elem 9 ]
        ]


test : Int
test =
    List.length
        <| List.filter ((==) False)
            [ flatten nl1 == List.range 1 9
            , flatten (SubList [ Elem 1, Elem 2 ]) == [ 1, 2 ]
            , flatten (SubList [ Elem "a", Elem "b" ]) == [ "a", "b" ]
            , flatten (SubList [ ]) == [ ]
            ]

Hints

1. You need to use the case ... of construct to handle SubList and Elem.
2. List.concatMap will be handy.

Solutions

Solutions