Elm Opaque Dictionary

Make dictionaries in Elm with arbitrary types for the keys.

Motivation

Elm's Dict allows you to make dictionaries with keys that have to be restricted to comparable types (Int, Float, Char, String, and tuples or lists of those).

But often you might want to make a dictionary where the key type is an opaque type, not a primitive. For example:

type UserId = UserId Int

It would be useful to have a user dictionary in which the keys are the user Ids, like so:

Dict UserId User

But you can't do that with Dict. The purpose of this module, OpaqueDict, is provide that functionality with a drop-in replacement for Dict. The main difference is that in a few places you have to provide a function that converts the key type to a string.

Examples

{-| Sample animal groups.
-}
type AnimalGroup
    = Feline
    | Canine
    | Rodent


{-| Get the string form of an animal group.
-}
animalGroupToString : AnimalGroup -> String
animalGroupToString group =
    case group of
        Feline ->
            "feline"

        Canine ->
            "canine"

        Rodent ->
            "rodent"

animalCounts : OpaqueDict AnimalGroup Int
animalCounts =
    [ ( Canine, 5 )
    , ( Feline, 3 )
    ]
        |> OpaqueDict.fromList animalGroupToString
        |> OpaqueDict.get Feline
        -- Just 3