This package exposes a really simple type called Id.

type Id x
    = Id String

Its for when your data has an id. Such as..

import Id exposing (Id)


type alias User =
    { id : Id ()
    , email : String
    }

Why an Id and not a String?

The Elm compiler is totally okay with the following code snippet..

viewUser : String -> String -> Html Msg
viewUser email id =
    -- first parameter is email
    -- second parameter is id




view : Model -> Html Msg
view model =
    div
        []
        [ viewUser
            -- woops! The parameters are mixed up
            model.user.id
            model.user.email
        ]

These mistake is really easy to make and they cause real problems, but if you just use an Id you can make them impossible.

Whats the x in Id x for?

You understand the problem in the previous example right? Here is a very similar problem..

type Id
    = Id String


updateUsersCatsFavoriteFood : Id -> Id -> Id -> Cmd Msg
updateUsersCatsFavoriteFood userId catId foodId =
    -- ..

Theres absolutely nothing stopping a developer from mixing up a catId with a userId or a foodId with a catId.

Instead we can do..

type Id x
    = Id String


updateUsersCatsFavoriteFood : Id User -> Id Cat -> Id Food -> Cmd Msg
updateUsersCatsFavoriteFood userId catId foodId =
    -- ..

Now with Id x, it is impossible (again) to mix up a Id User with a Id Cat. They have different types. And the compiler will point out if you try and use a Id User where only a Id Cat works.

Okay, there is one trade off

The following code is not possible due to a circular definition of User..

type alias User =
    { id : Id User }

Easy work arounds include..

type UserId
    = UserId (Id User)


type alias User =
    { id : UserId }

and

type User
    = User { id : Id User }

..but I would encourage you to build your architecture such that data does not contain its own Id x to begin with. Instead, get used to operating on (Id User, User) pairs, and treat the left side as the single source of truth for that identifier.

    ( Id User, User )

Id