Problem 8

Write a function to remove consecutive duplicates of list elements.

Example

``````noDupes [1, 1, 2, 2, 3, 3, 3, 4, 5, 4, 4, 4, 4]
== [1, 2, 3, 4, 5, 4]
``````

Unit Test

``````import Html
import List
import Maybe

noDupes : List a -> List a
noDupes xs =
[]

main : Html.Html a
main =
Html.text
<| case test of
0 ->

1 ->

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

test : Int
test =
List.length
<| List.filter ((==) False)
[ noDupes [ 1, 1, 1, 1, 2, 5, 5, 2, 1 ] == [ 1, 2, 5, 2, 1 ]
, noDupes [ 2, 1, 1, 1 ] == [ 2, 1 ]
, noDupes [ 2, 2, 2, 1, 1, 1 ] == [ 2, 1 ]
, noDupes [ 1 ] == [ 1 ]
, noDupes [] == []
, noDupes [ "aa", "aa", "aa" ] == [ "aa" ]
, noDupes [ "aab", "b", "b", "aa" ] == [ "aab", "b", "aa" ]
]
``````

Hints

1. How would you copy a list using `foldr`? How would you modify the function passed to `foldr` only copy new values?
2. How would you drop the head of the list if it's a duplicate? Can you apply that recursively to solve the problem?

Solutions