nicklawls/list-selection

A list with a selected item. Like a zipper, but optional.

Selection List

Create a list which may have one (but no more than one) item selected. You can never select an item that isn't in the list. Selections are optional, so this is slightly different than a zipper.

Usage

Create a Selection list from a regular list by using fromList. Let's use this to choose what we'd like for lunch.

import List.Selection exposing (Selection)


type Lunch
    = Burrito
    | ChickenWrap
    | TacoSalad
    | DonerKebab


todaysMenu : Selection Lunch
todaysMenu =
    [ Burrito, ChickenWrap, TacoSalad ]
        |> List.Selection.fromList       -- create a new Selection list
        |> List.Selection.select Burrito -- now let's see, I think I'd like a burrito (yum, monads!)

Since I already chose what I want for lunch, I can get it with selected:

List.Selection.selected todaysMenu -- `Just Burrito`

But what if you try and select something that doesn't exist in the list? The shop was out of doner kebab today, but what if we ask for it?

todaysMenu
    |> List.Selection.select DonerKebab -- this doesn't exist in our menu, so...
    |> List.Selection.selected          -- `Just Burrito` (selection unchanged)

And if I change my mind, I can remove my choice with deselect:

todaysMenu
    |> List.Selection.deselect -- deselect any current selection
    |> List.Selection.selected -- `Nothing`

Developing

Install Elm and elm-test and elm-verify-examples from NPM, then run make to run tests and generate documentation.

License

Licensed under a BSD 3-Clause license