(Disclaimer: This post was written about Elm 0.16. Signals have since been deprecated. The concepts in this post may still help understand how the Elm Architecture works internally, but the actual code has changed significantly)

This is the second post in a series on some of the concepts in Elm that might be a bit puzzling when you start out with Elm. In the last post about Signals in Elm I wrote about Signals and how they are behind the scenes of StartApp.Simple. In this post I get into long running operations like XHRs (aka AJAX). There are two closely related types that are involved in this, Tasks and Effects, and the exact differences between can be confusing in the beginning. So let’s dive right in:

Tasks

Task is the more basic type (it is defined in Core) and so let’s start with this one. A Task represents a long-running operation that can fail (with an error type) or succeed (with a success type). It’s type is thus:

Task errorType successType

-- (or, as it is actually written in the library:)

Task x a

There are only two values of Task you can create yourself in your code, without using a separate library. These two ways are two functions:

succeed : a -> Task x a

-- and

fail : x -> Task x a

These are ways to create task values without actually doing any long-running operations. This can be useful if you want to combine a long running task and a simple value in some way and process them further (you would then turn the simple value into a Task with succeed).

Most of the time you actually get in contact with tasks, these will be created for you by library functions that initialize the task so that it will perform some long running operation when the runtime executes it and handle the result of the operation according to Task semantics (I.e. that some native code will make sure to call fail or succed on the native representation of the task when the operation is finished).

Note that the operation doesn’t start right away! I said “when the runtime executes it”. In purely functional programming languages, inside the language you can never just perform a side effect (something that changes “the world” outside the internal state of your code), and sending an http request surely is a side effect in that sense. This is one of the big mental shifts from imperative programming, where you can always do this, to working with purely functional languages where you can’t.

So how does this actually work? The task you get back represents the long running action. When a library creates it for you, nothing “happens”, it just created a value (of type Task) that indicates what you would like to happen.

If you look at e.g. the implementation of the native part of the send function in http you will see that a Task is created in native JS code by handing it a callback. This callback is what will get called when the runtime actually executes the task. This is when the actual XHR request is created and performed - only when the runtime executes this callback.

So how do you get the runtime to run this task? By passing it into an outgoing port. I will go into detail on ports in a later blogpost, but suffice to say that they are a way to send messages between “native” JS and Elm (called an incoming port) and from Elm to native JS (an outgoing port). The runtime has some special casing for Tasks that come to it via outgoing ports that make it execute the callback the Task represents.

When the long running native code is done, it will either call succeed or fail on the task. In most real life code that uses the Elm Architecture you will set up a “chain” of task processing that will lead to the the end result of the task execution being that a value of your Action type is routed back through your update function. This value of your Action type is the usually tagged with the result of the task (e.g. the decoded Json response of an XHR).

As a last piece of info before we have a look at Effects and how all of this actually looks in an example, let me just mention that Tasks can easily be chained togehter with andThen, much like promises in JS are chained together.

Effects

On to Effects! If you look at the definition for Effects it’s pretty simple:

type Effects a
    = Task (Task.Task Never a)
    | Tick (Time -> a)
    | None
    | Batch (List (Effects a))

None and Batch are helpers, so the basic things an Effect can represent are Tasks (with error type Never) and Ticks. The latter is used for animations if you want to do something at the next animation frame.

It’s very common to turn a Task into an Effect, whereas the inverse is usually only ever done by StartApp/the runtime.

Several libraries use Task to allow you to work with long running operations - Http is one, elm-history is another.

So how is this used? The Elm Architecture example 5 uses this very central piece of code:

getRandomGif : String -> Effects Action
getRandomGif topic =
  Http.get decodeImageUrl (randomUrl topic)
    |> Task.toMaybe
    |> Task.map NewGif
    |> Effects.task

Let’s look at what it does. It starts with creating a task that represents the Http get operation and then builds a chain on top of this. I will deconstruct this from using the pipe operator to normal function calls with type annotations to hopefully explain what’s happening:

getRandomGif topic =
  getTaskWithError : Task Http.Error String
  getTaskWithError = Http.get decodeImageUrl (randomUrl topic)

  getTaskWithNoErrorAndMaybe : Task Never (Maybe String)
  getTaskWithNoErrorAndMaybe = Task.toMaybe getTaskWithError

  getTaskWithNoErrorAndAction : Task Never (Action)
  getTaskWithNoErrorAndAction = Task.map NewGif getTaskWithNoErrorAndMaybe

  taskAsEffect : Effects Action
  Effects.task getTaskWithNoErrorAndAction

So in the end, Effects in this case just wraps the Task for us. Because we used toMaybe and then mapped it to the NewGif type constructor function, this will result in an Action coming back to us via update when it is done that is either (NewGif Nothing) if the http request failed, or (NewGif "some-url-here") if it succeeds. If you want to understand how this wiring happens I would suggest looking at the implementation of Effects.

One thing that is worth looking at is the return type of the function: Effects Action. Effects has a type variable, just like for example List. So this is an Effects that deals with the Action type you define in your application - and this is the really neat part of how to make sure that you can deal with the result of the Task/Effect - the result will just be a value of your Action type!

At this point you may wonder: why have Effects at all? Aren’t they just weird wrappers for Tasks? Let’s quickly take a look again at how the Task case of the Effects type is defined:

type Effects a
    = Task (Task.Task Never a)

The way StartApp is typed, going via Effects Action constraints all Tasks to come back with Actions and have the error type Never. This is really nice because it means that you don’t have to handle different task types to different ports, but you set up “pipelines” of tasks to effects like with getRandomGif above and it will all work out in a typesafe manner that the result of your tasks will be sent back to your program’s update function as Actions.

Ok, so finally, remember I wrote something about having to send Tasks off to an outgoing port. I you don’t do this, the Tasks will never be executed! If you want to use Effects, the easiest way is to switch from StartApp.Simple to StartApp. This brings three minor changes with it:

  1. start no longer directly returns a Signal of Html but a record of 3 Signals: one for the html, one for the model, and, crucially for us, one of tasks.
  2. The update function now returns not just the Model, but a tuple of (Model, Effects Action). I.e. that every case in your update function will have to return both the changed model and an Effect (which will often be Effects.none)
  3. start gets a fourth parameter, inputs, for incoming Signals.
  4. The tasks part of the record that is returned by start has to be handed to a port so that the Tasks/Effects are actually performed by the runtime like so:
app =
  StartApp.start
    { init = init "funny cats"
    , update = update
    , view = view
    , inputs = []
    }


main =
  app.html


port tasks : Signal (Task.Task Never ())
port tasks =
  app.tasks

Phew, this got a little long again, but I hope it helps a little to understand how Tasks/Effects work!