An Elm context type that can be run everywhere. Like a Reader.

Context.elm

module Context exposing (..)


type Context a b
    = WithContext (a -> b)


withContext : (a -> b) -> Context a b
withContext =
    WithContext


runContext : Context a b -> a -> b
runContext (WithContext fn) a =
    fn a

Suppose you have a api context:

type alias ApiEnv a =
    { a | api : { url : String } }

then you can define your requests for example like

upload : File.File -> (Result Http.Error String -> msg) -> Context (ApiEnv a) (Cmd msg)
upload file uploaded =
    withContext
        (\ctx ->
            Http.request
                { method = "POST"
                , headers = []
                , url = ctx.api.url ++ "/uploads"
                , body =
                    Http.multipartBody
                        [ Http.filePart "file" file
                        ]
                , expect = Http.expectString uploaded
                , timeout = Nothing
                , tracker = Just (File.name file)
                }
        )

and in your update function when you call the command:

 runContext (Api.upload file (Uploaded << Result.mapError HttpError)) model.env -- :: Cmd Msg

where model.env incorporates the composed AppEnv.

Upon further discussion, this actually is a Reader. 😄

See Punie/elm-reader

With open records as environment, the environments can be composed and scoped.