Problem 1

Write a function last that returns the last element of a list. An empty list doesn't have a last element, therefore last must return a Maybe.

Example

Cut and paste the following code into the online compiler http://elm-lang.org/try. Insert your implementation of last and test.

import Html exposing (text)
import Maybe


last : List a -> Maybe a
last xs = 
  -- your implementation goes here
  Nothing


main =
    case last [ 1, 2, 3, 4 ] of
        Just a ->
            text (toString a)

        Nothing ->
            text "No element found"

Result:

4

Unit Test

Let's create a more complete test, checking last against an empty list and lists of different types. We will use assertEqual which takes a tuple, where the first element will hold a test condition and the second holds the expected result. This will allow us to test multiple conditions concisely.

Cut and paste the following code into the online compiler http://elm-lang.org/try. Insert your implementation of last and test.

import Html
import List
import Maybe


last : List a -> Maybe a
last xs =
    -- your implementation goes here
    Nothing


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."


test : Int
test =
    List.length
        <| List.filter ((==) False)
            [ last (1..4) == Just 4
            , last [ 1 ] == Just 1
            , last [] == Nothing
            , last [ 'a', 'b', 'c' ] == Just 'c'
            ]


(..) : Int -> Int -> List Int
(..) start end =
    List.range start end

Hints

Here are 2 hints to solve the problem:

1. Use recursion.
2. How can you leverage a List's head function to solve this problem?

Solutions

After you've solved the problem, check out these solutions.