Elm Update Pipeline

A library for sequential composition of updates in the convenient style of pipelines, where functions are chained together using the pipe operator. For example;

update msg model =
    case msg of
        SomeMsg someMsg ->
            save model
                |> andThen (setPower 100)
                |> andAddCmd someCmd

Monadic functions of type a -> ( b, Cmd msg ) form the building blocks of a pipeline. We use save to create an update without any commands, and andThen to extract the model from a result and pass it as input to the next function in the pipeline.

showToast : String -> Model -> ( Model, Cmd msg )
showToast = ...

setColor : Color -> Model -> ( Model, Cmd msg )
setColor color model =
    save { model | color = color }

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    model
        |> setColor Orange
        |> andThen (showToast "Color changed to orange")

The applicative interface, map2, map3, etc., together with andMap, addresses the need to map functions with more than one parameter over ( model, Cmd msg ) inputs.

type alias Model =
    { menuOpen : Bool, session : Session, router : Router.Model }

initSession : Flags -> ( Session, Cmd Msg )
initSession = ...

init : Flags -> ( Model, Cmd Msg )
init flags =
    save Model
        |> andMap (save False)
        |> andMap (initSession flags)
        |> andMap initRouter

In this example, init can also be defined as

init flags =
    map3 Model (save False) (initSession flags) initRouter