A list of items, one of which might be selected.

Create a Selection a with nothing selected.

Convert a Selection list back to a regular list. This is useful for creating view functions, for example. If you want a list that has the selected item, use mapSelected like this:

[ 1, 2, 3 ]
    |> fromList
    |> select 2
    |> mapSelected
        { selected = \a -> (True, a)
        , rest = \a -> (False, a)
        }
    |> toList
    --> [ (False, 1), (True, 2), (False, 3) ]

Mark an item as selected. This will select at most one item. Any previously selected item will be unselected.

fromList ["Burrito", "Chicken Wrap", "Taco Salad"]
    |> select "Burrito"
    |> selected --> Just "Burrito"

Attempting to select an item that doesn't exist is a no-op.

fromList ["Burrito", "Chicken Wrap", "Taco Salad"]
    |> select "Doner Kebab"
    |> selected --> Nothing

Mark an item as selected by specifying a function. This will select the first item for which the function returns True. Any previously selected item will be unselected.

fromList ["Burrito", "Chicken Wrap", "Taco Salad"]
    |> selectBy (String.startsWith "B")
    |> selected --> Just "Burrito"

Deselect any selected item. This is a no-op if nothing is selected in the first place.

Get the selected item, which might not exist.

fromList ["Burrito", "Chicken Wrap", "Taco Salad"]
    |> select "Burrito"
    |> selected --> Just "Burrito"

Apply a function to all the items.

fromList [1, 2, 3]
    |> map ((*) 2)
    |> toList --> [2, 4, 6]

Apply a function to all the items, treating the selected item specially.

fromList [1, 2, 3]
    |> select 2
    |> mapSelected { selected = (*) 2, rest = identity }
    |> toList --> [1, 4, 3]

Filter all items where predicate evaluates to false, preserving selected item when unfiltered.

fromList [1, 2, 3]
    |> select 2
    |> filter ((>) 2)
    |> toList --> [1]