elm-task-parallel

Run tasks in parallel and handle the results only if every task finishes successfully, similar to Promise.all() in Javascript.

See the examples folder for full code examples.

Motivation

It is common to have several tasks where the results only matter together. For example, you may need to complete multiple HTTP requests before your page can be rendered. In order to avoid running the tasks in sequence which is slower, you typically have to

• Batch task commands together
• Handle each task's success case
• Handle each task's error case
• Check if each one is finished every time an individual task finishes

This library is designed to do that for you.

How to use

Instead of using Task.attempt, use one of the helper functions to run up to 5 tasks of different result types (or a list of the same type). It will return a tuple with some internal state and a command.

( internalState, fetchCmd ) =
        attempt5
            RequestsUpdated
            Api.fetchUser
            Api.fetchOptions
            Api.fetchLocations
            Api.fetchChat
            Time.now

Store the state and pass along the command. Your model will need to store a State type matching the number of your tasks.

State5 User Options Locations Chat Time.Posix

The message you passed in to the helper function will need to accept an internal Msg with a similar type annotation, but including the error type to handle.

type Msg
    = RequestsUpdated (Msg5 Http.Error User Options Locations Chat Time.Posix)
    | RequestsFinished User Options Locations Chat Time.Posix
    | RequestFailed Http.Error

and finally your update function will only need to handle three cases

• Internal updates where you again store the state and pass the command along
• The success case where all of the tasks have successfully completed
• The error case of the first task to fail

case msg of
    RequestsUpdated internalMsg ->
        let
            ( nextState, nextCmd ) =
                update5 model.internalState internalMsg RequestsFinished RequestFailed
        in
        ( { model | internalState = nextState }, nextCmd )

    RequestsFinished user options locations chat time ->
        ( { model | user = user, options = , locations = locations, chat = chat, currentTime = time }, Cmd.none )
        
    RequestFailed err ->
        ( { model | maybeError = Just err, Cmd.none )

Caveats

• If the tasks have different result types, you're limited to 5 tasks (mirroring Maybe.map). For HTTP requests, this is a limit I haven't run into yet. For lists of tasks, there is no limit.
• Updating the internal state of this library adds one case to your update function, however in the case of 5 tasks you could already have 10 cases just to update those + a completion check. This library limits those to just three.