Making complex update functions with ccapndave/elm-update-extra

Introduction#

ccapndave/elm-update-extra is a fantastic package which helps you handle more complex updating functions, and may be very useful.

Message which call a list of messages

Using sequence function you can easily describe a message that calls a list of other messages. It’s useful when dealing with semantics of your messages.

Example 1: You are making a game engine, and you need to refresh the screen on every frame.

module Video exposing (..)
type Message = module Video exposing (..)

import Update.Extra exposing (sequence)

-- Model definition [...]

type Message
    = ClearBuffer
    | DrawToBuffer
    | UpdateLogic
    | Update

update : Message -> Model -> (Model, Cmd)
update msg model =
    case msg of
        ClearBuffer ->
            -- do something
        DrawToBuffer ->
            -- do something
        UpdateLogic ->
            -- do something
        Update ->
            model ! []
                |> sequence update [ ClearBuffer
                                   , DrawToBuffer
                                   , UpdateLogic]

Chaining messages with andThen

The andThen function allows update call composition. Can be used with the pipeline operator (|>) to chain updates.

Example: You are making a document editor, and you want that each modification message you send to your document, you also save it:

import Update.Extra exposing (andThen)
import Update.Extra.Infix exposing (..)

-- type alias Model = [...]

type Message
    = ModifyDocumentWithSomeSettings
    | ModifyDocumentWithOtherSettings
    | SaveDocument

update : Model -> Message -> (Model, Cmd)
update model msg =
    case msg of
        ModifyDocumentWithSomeSettings ->
            -- make the modifications
            (modifiedModel, Cmd.none)
            |> andThen SaveDocument
        ModifyDocumentWithOtherSettings ->
            -- make other modifications
            (modifiedModel, Cmd.none)
            |> andThen SaveDocument
        SaveDocument ->
            -- save document code

If you import also Update.Extra.Infix exposing (..) you may be able to use the infix operator:

update : Model -> Message -> (Model, Cmd)
update model msg =
    case msg of
        ModifyDocumentWithSomeSettings ->
            -- make the modifications
            (modifiedModel, Cmd.none)
            :> andThen SaveDocument
        ModifyDocumentWithOtherSettings ->
            -- make other modifications
            (modifiedModel, Cmd.none)
            :> SaveDocument
        SaveDocument ->
            -- save document code