Non empty lists using Zippers

Search…

Problem
Solution
1
type alias Book =
2
    List Page
3
​
4
currentPage : Book -> Page
5
currentPage book =
6
    case book of
7
        page :: _ ->
8
            page
9
        [] ->
10
            Debug.todo "this is a dead end."
11
   
Copied!
1
type alias Book =
2
    { previous : List Page
3
    , current : Page
4
    , next : List Page
5
    }
6
​
7
currentPage : Book -> Page
8
currentPage book =
9
    book.current
Copied!
Question
How can I  ensure that a list always contains at least one element.
Answer
Such a list is called a Zipper List:
1
type alias ZipperList a =
2
    { previous : List a
3
    , current : a
4
    , next : List a
5
    }
Copied!
Package: Use miyamoen/select-list for a complete implementation of a Zipper List.
Further reading
📦Package: miyamoen/select-list​
📦Package: wernerdegroot/listzipper​

Recipes - Previous

Making impossible states Impossible

Restrict records using Opaque Types

Last modified 2yr ago

Copy link

Contents

Question

Answer