Id

This package exposes a really simple type called Id. Its for when your data has an id. Such as..

import Id exposing (Id)

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

You can make Ids from strings..

import Id exposing (Id)

user : User
user = 
    { id = Id.fromString "400" 
    , email = "[email protected]"
    }

Or you can generate random Ids..

import Id exposing (Id)
import Random.Pcg as Random exposing (Seed)


user : Seed -> ( User, Seed )
user seed =
    let
        ( id, nextSeed ) =
            Random.step Id.generator seed
    in
    ( { id = id, email = "[email protected]" }
    , nextSeed
    )

Why use an Id instead of a String? Technically, all of the following are possible with String ids..

coolString : User -> String
coolString user =
    user.email ++ user.id


screwUpUser : User -> User
screwUpUser user =
    { user
        | id = user.email
        , email = user.id
    }


henry : User
henry =
    User "Henry" henrysId

Admittedly these errors seem implausible, but using a separate Id type eliminates them entirely. Theres really no functionality lost either; in fact, the whole point is that there shouldnt be any functionality with ids. Ids largely just get set and then forgotten about. Using this Id type ensures your expectations.

And then of course you can decode and encode Ids too..

Encode.encode 0 (Id.encode id)
-- "\"hDFL0Cs2EqWJ4jc3kMtOrKdEUTWh\""

Decode.decodeString (Decode.field "id" Id.decoder) "{\"id\":\"19\"}"
-- Ok (Id 19) : Result String Id

Origin Type

I am going to describe an app that sounds like something you might have made before. It revolves around a specific data type (say a Car), and it has one page where you can create a new Car and another where you can edit existing Cars. A Car is what is it, but if its being updated it already exists and therefore has an id, while if its new it necessarily doesnt exist and doesnt have an id. How do you represent that? Heres an idea..

type Origin
    = Local
    | Remote Id

type alias Car =
    { origin : Origin
    , make : String
    }

saveCar : Car -> Http.Request (Result Err ())
saveCar car =
    case car.origin of
        Id id ->
            Car.update id car

        New ->
            Car.create car