Return

When writing Elm code you unavoidably have to interact with types like this:

(model, Cmd msg)

So we make that a type:

type alias Response msg model = (model, Cmd msg)

As it turns out, Response is really Haskell's Writer since Cmd msg is a Monoid. Basically this means the functions exposed in this library allow you to interact with (model, Cmd msg) using familiar functions like map, map2, and andThen, without having to worry about Cmd's (unless you opt in) as they get Cmd.batched together automatically.

Static piping

x : Return Msg Model
x = y >>| \model -> tell (doThing model.baz)
       >| foo model.baa
      >>| bar >> singleton
        |> map (\model -> {model | baa = 4})

Without static piping, we are forced to use more language features, and the intention of our code is arguably less clear or sequential.

x' : Return Msg Model
x' = case y of
  (model, cmd) ->
    let
      (model', cmd') = foo model.baa
      (model'') = bar model'
    in {model'' | baa = 4} ! [cmd, doThing model.baz, cmd']

Give the following types for these examples

type alias Model = { baz : String, baa : Int }
y : Return Msg Model
doThing : String -> Cmd Msg
foo : Int -> Return Msg Model
bar : Model -> Model