Non empty lists using Zippers

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

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