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elmunion-types

Elm union subsets

Say I have a union type like this:

type Route
  = Home
  | License
  | UserProfile { username : String }
  | Search { query : String }
  | SomeOtherPage

In practice I frequently need to work with subsets of this union. For example:

type StaticRoute = Home | License

I would like to be able to define functions which accept subsets like the above, instead of the wider Route.

I don't want to nest StaticRoute inside of Route, like so:

type Route
  = Static StaticRoute
  | UserProfile { username : String }
  | Search { query : String }
  | SomeOtherPage

This is because I want to be able to define many different subsets of Route, some of which could overlap:

type StaticRoute = Home | License
type RouteWithServerRendering = Home | Search { query : String }
type LoggedInRoute = SomeOtherPage
-- and so on…

How then can I define subsets of Route without repeating definitions?

Solution

  • Jasper Woudenberg recently posted Conversion functions, five stars , which advocates for having similar types and using conversion functions to translate between one type to another.

    In your case, it might look like this:

    module Route exposing (fromStaticRoute, toStaticRoute)

fromStaticRoute : StaticRoute -> Route
fromStaticRoute staticRoute =
    case staticRoute of
        Static.Home ->
            Home
        Static.License ->
            License

toStaticRoute : Route -> Maybe StaticRoute
toStaticRoute route =
    case route of
        Home ->
            Just Static.Home
        License ->
            Just Static.License
        _ ->
            Nothing