99 PSiE [Problem 5]

はじめに

Ninety-nine Problems, Solved in Elm を毎日さわることで､Elm に親しもうという『日記』です｡詳細は 99 PSiE [Problem 1] をご覧ください｡

問題

Elm provides the function List.reverse to reverse the order of elements in a list. See if you can implement it.
(引用元: Problem 5)

成果物

import Html

main : Html.Html a
main =
    Html.text
        <| case test of
            0 -> "Your implementation passed all tests."
            1 -> "Your implementation failed one test."
            n -> "Your implementation failed " ++ Debug.toString n ++ " tests."


test : Int
test =
    List.length
        <| List.filter ((==) False)
            [ myReverse_ [1, 2, 3, 4] == [4, 3, 2, 1]
            , myReverse_ [2, 1] == [1, 2]
            , myReverse_ [1] == [1]
            , myReverse_ [] == []
            , myReverse_ [ 'a', 'b', 'c' ] == [ 'c', 'b', 'a' ]
            ]

-- My solution 1
myReverse list =
    case list of
        [] -> list
        x::xs -> myReverse xs ++ [x]

-- My solution 2
myReverse_ list = List.foldr (\y ys -> ys ++ [y]) [] list

正解

=> Problem 5 Solutions

コメント

List.foldr と List.foldl の良い勉強になった｡