Elm Concurrent Task

What?

• An alternative Task api - run a tree of tasks concurrently.
• A hack free implementation of Task Ports - call JavaScript functions as tasks.
• Run anywhere - works in the Browser or NodeJS.

This package is heavily inspired by elm-pages' BackendTask and is intended to be a standalone implementation that can be dropped into any Elm app - big kudos to Dillon for the idea.

See the examples for more things you can do!

View the elm-package docs here.

Why?

Structured Concurrency

Task.map2, Task.map3+ In elm/core run each subtask in sequence.

Whilst it's possible to run these subtasks concurrently as separate Cmds, it can be a lot of wiring and boilerplate, including:

• Batching task commands together.
• Handling each task's success case.
• Handling each task's error case.
• Checking if all other tasks are completed every time an individual task finishes.

Elm Task Parallel handles this nicely but only at the top level (sub tasks cannot be parallelised).

And what if you want to speed up a complex nested task like:

Task.map2 combine
    (task1
        |> Task.andThen task2
        |> Task.andThen
            (\res ->
                Task.map2 combine
                    (task3 res)
                    task4
            )
    )
    (Task.map2 combine
        task5
        task6
    )

This is the elm equivalent of "callback hell".

This library helps you do this with a lot less boilerplate.

A Sequenceable JavaScript FFI

Sometimes you want to call JavaScript from elm in order. For example sequencing updates to localstorage:

NOTE: See a full working localstorage example here.

import ConcurrentTask exposing (ConcurrentTask)
import Json.Decode as Decode exposing (Decoder)
import Json.Encode as Encode



-- Preferences


type alias Preferences =
    { contrast : Int
    , brightness : Int
    }


setContrast : Int -> ConcurrentTask Error ()
setContrast contrast =
    getItem "preferences" decodePreferences
        |> ConcurrentTask.map (\preferences -> { preferences | contrast = contrast })
        |> ConcurrentTask.andThen (encodePreferences >> setItem "preferences")


encodePreferences : Preferences -> Encode.Value
encodePreferences p =
    Encode.object
        [ ( "contrast", Encode.int p.contrast )
        , ( "brightness", Encode.int p.brightness )
        ]


decodePreferences : Decoder Preferences
decodePreferences =
    Decode.map2 Preferences
        (Decode.field "contrast" Decode.int)
        (Decode.field "brightness" Decode.int)



-- Localstorage


type Error
    = NoValue
    | ReadBlocked
    | DecodeError Decode.Error
    | WriteError String


getItem : String -> Decoder a -> ConcurrentTask Error a
getItem key decoder =
    ConcurrentTask.define
        { function = "localstorage:getItem"
        , expect = ConcurrentTask.expectString
        , errors = ConcurrentTask.expectErrors decodeReadErrors
        , args = Encode.object [ ( "key", Encode.string key ) ]
        }
        |> ConcurrentTask.map (Decode.decodeString decoder >> Result.mapError DecodeError)
        |> ConcurrentTask.andThen ConcurrentTask.fromResult


setItem : String -> Encode.Value -> ConcurrentTask Error ()
setItem key value =
    ConcurrentTask.define
        { function = "localstorage:setItem"
        , expect = ConcurrentTask.expectWhatever
        , errors = ConcurrentTask.expectThrows WriteError
        , args =
            Encode.object
                [ ( "key", Encode.string key )
                , ( "value", Encode.string (Encode.encode 0 value) )
                ]
        }


decodeReadErrors : Decoder Error
decodeReadErrors =
    Decode.string
        |> Decode.andThen
            (\reason ->
                case reason of
                    "NO_VALUE" ->
                        Decode.succeed NoValue

                    "READ_BLOCKED" ->
                        Decode.succeed ReadBlocked

                    _ ->
                        Decode.fail ("Unrecognized Read Error: " ++ reason)
            )

Hack Free you say?

Other implementations of Task Ports rely on either:

• ServiceWorkers - intercept certain http requests and call custom JavaScript from the service worker.
• Monkeypatching XMLHttpRequest - Modify methods on the global XMLHttpRequest to intercept http requests and call custom JavaScript.

Both methods are not ideal (modifying global methods is pretty dodgy), and neither are portable to other environments like node (ServiceWorker and XMLHttpRequest are only native in the browser and require pollyfills).

It's just ports!

elm-concurrent-task uses plain ports and a bit of wiring to create a nice Task api.

This makes it dependency free - so more portable (🤓) and less likely to break (😄).

Caveats

Because elm-concurrent-task uses a different type to elm/core Task it's unfortunately not compatible with elm/core Tasks.

However, there are a number of tasks built into the JavaScript runner and supporting modules that should cover a large amount of the existing functionality of elm/core Tasks.

Check out the built-ins for more details:

• Browser.Dom
• Http
• Process
• Random
• Time

How?

Getting Started

1. Install Elm and JavaScript/TypeScript packages

Install the elm package with

elm install andrewMacmurray/elm-concurrent-task

Install the JavaScript/TypeScript runner with

npm install @andrewmacmurray/elm-concurrent-task

2. Add to your Elm app

Your Elm program needs:

• A single ConcurrentTask.Pool in your Model to keep track of each task attempt:

  type alias Model =
      { tasks : ConcurrentTask.Pool Msg Error Success
      }
  

• 2 Msgs to handle task updates:

  type Msg
      = OnProgress ( ConcurrentTask.Pool Msg Error Success, Cmd Msg ) -- updates task progress
      | OnComplete (ConcurrentTask.Response Error Success) -- called when a task completes
  

• 2 ports with the following signatures:

  port send : Decode.Value -> Cmd msg
  port receive : (Decode.Value -> msg) -> Sub msg
  

Here's a simple complete program that fetches 3 resources concurrently:

port module Example exposing (main)

import ConcurrentTask exposing (ConcurrentTask)
import ConcurrentTask.Http as Http
import Json.Decode as Decode


type alias Model =
    { tasks : ConcurrentTask.Pool Msg Error Titles
    }


type Msg
    = OnProgress ( ConcurrentTask.Pool Msg Error Titles, Cmd Msg )
    | OnComplete (ConcurrentTask.Response Error Titles)


type alias Error =
    Http.Error



-- Get All Titles Task


type alias Titles =
    { todo : String
    , post : String
    , album : String
    }


getAllTitles : ConcurrentTask Error Titles
getAllTitles =
    ConcurrentTask.succeed Titles
        |> ConcurrentTask.andMap (getTitle "/todos/1")
        |> ConcurrentTask.andMap (getTitle "/posts/1")
        |> ConcurrentTask.andMap (getTitle "/albums/1")


getTitle : String -> ConcurrentTask Error String
getTitle path =
    Http.get
        { url = "https://jsonplaceholder.typicode.com" ++ path
        , headers = []
        , expect = Http.expectJson (Decode.field "title" Decode.string)
        , timeout = Nothing
        }



-- Program


init : ( Model, Cmd Msg )
init =
    let
        ( tasks, cmd ) =
            ConcurrentTask.attempt
                { send = send
                , pool = ConcurrentTask.pool
                , onComplete = OnComplete
                }
                getAllTitles
    in
    ( { tasks = tasks }, cmd )


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        OnComplete response ->
            let
                _ =
                    Debug.log "response" response
            in
            ( model, Cmd.none )

        OnProgress ( tasks, cmd ) ->
            ( { model | tasks = tasks }, cmd )


subscriptions : Model -> Sub Msg
subscriptions model =
    ConcurrentTask.onProgress
        { send = send
        , receive = receive
        , onProgress = OnProgress
        }
        model.tasks


port send : Decode.Value -> Cmd msg


port receive : (Decode.Value -> msg) -> Sub msg


main : Program {} Model Msg
main =
    Platform.worker
        { init = always init
        , update = update
        , subscriptions = subscriptions
        }

3. Register the runner in your JavaScript/TypeScript app

Connect the runner to your Elm app (the runner supports both import and require syntax):

import * as ConcurrentTask from "@andrewmacmurray/elm-concurrent-task";

const app = Elm.Main.init({});

ConcurrentTask.register({
  tasks: {},
  ports: {
    send: app.ports.send,
    receive: app.ports.receive,
  },
});

The value passed to tasks is an object of task names to functions (the functions can return plain synchronous values or promises)

e.g. tasks for reading and writing to localStorage:

const tasks = {
  "localstorage:getItem": (args) => localStorage.getItem(args.key),
  "localstorage:setItem": (args) => localStorage.setItem(args.key, args.item),
};

NOTE: for a more complete localStorage integration with proper error handling check out the localstorage example.

Re-using ports

Each send and receive port pair only support one ConcurrentTask.Pool subscribed at a time. Weird things can happen if you have two or more ConcurrentTask.Pools using the same ports at the same time.

Generally this should not be needed, but if you have a use-case, please leave an issue.

Develop Locally

Install Dependencies:

npm install

Run the tests with:

npm test

To preview any changes, try some of the examples in the examples folder.

View the docs locally with:

npm run docs

Publishing a new release

1. Run elm bump to bump the elm version.
2. Update the version in package.json to match the new elm version.
3. Ensure all links in the README / docs are pointing to the correct new version.
4. Commit and push the updates.
5. Wait for the Publish Github Action to complete.