elm-keyword-list

A list of values of any type, which can also be lists.

The main use case of this library is managing list of optional and nested values, for example, like Html nodes or Html.Attributes. Usually, we can solve this issue using List (List any) or List (Maybe any), like this:

myView : Bool -> Html msg
myView isHidden =
    Html.div []
        (List.concat
            [ [ Html.text "Always visible"
              , Html.text "Also always visible"
              ]
            , if isHidden then
                []
              else
                [ Html.text "Maybe visible" ]
            , if not isHidden then
                [ Html.text "Maybe not visible" ]
              else
                []
            ]
        )

Using KeywordList, the above code will look like this:

myView : Bool -> Html msg
myView isHidden =
    Html.div []
        (KeywordList.fromMany
            [ KeywordList.one (Html.text "Always visible")
            , KeywordList.one (Html.text "Always visible")
            , KeywordList.ifTrue isHidden (Html.text "Maybe visible")
            , KeywordList.ifFalse isHidden (Html.text "Maybe not visible")
            , KeywordList many
                [ Html.text "Even nested list, without calling again fromMany"
                ]
            ]
        )

A Note About Performance

This library optimizes converting KeywordList back to a list in linear time. Here is a benchmark comparing using KeywordList against using List and List.concat.