(Disclaimer: This post was written about Elm 0.16. Signals, the mechanism described in this post, 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 third post in a series of posts about Elm. In my first post about Signals in Elm I briefly mentioned ports. Since they are the only way to communicate with “native” Javascript, they certainly warrant a closer look. If you haven’t checked out the last post in this series on tasks and effects I suggest you do that now as this post will build on these concepts.

So what are Ports, exactly? They are basically a way to send messages from Elm to native JS or from JS to Elm. They are defined in Elm with their own keyword, port. If a Port is defined to be of a Non-Signal type (e.g. port initialUrl : String) then it is a “one time” message (at init time of the Elm code), i.e. such ports can be used if you want to send initialization values from JS to Elm at init time (and never afterwards). More frequently it will be a Signal of some type (e.g. a Signal String). Ports can not send and receive values of any type but only a subset - the big two groups of values that can’t be used are functions and union types (Maybe is the only exception to this rule). All the details can be found on the elm guide page on interop.

As a simple example, let’s create a pair of ports to send Strings from Elm to Js, do something with it in native JS, and then send the Strings back. One case where something like this might be useful would be to encrypt values using a native library (using one of the new Browser crypto APIs).

Outgoing ports

Since we want to send stuff several times, not just once, we need to declare both ports as Signals. Let’s start with the outgoing port and on the Elm side:

port requestEncryption : Signal String
port requestEncryption =
  -- how do we get a Signal here for the implementation?

This is a simple outgoing port definition, but somehow we have to implement this port - we need a way to get a Signal that we can “trigger” in our elm app and that will be received on the JS side. Let’s see how the JS side will look like before coming back to Elm:

var div = document.getElementById('root');
var myapp = Elm.embed(Elm.MyApp, div);
myapp.ports.requestEncryption.subscribe(encryptString);

function encryptString(message) {
    // do something with the string, send it to be encrypted etc
}

Pretty straigthforward. We use the subscribe method to register a callback that will be called with the message string value every time the Signal fires at the Elm side. Ok, but how do we finish implementing this on the Elm side?

There are two ways to do this - one is to create a new Mailbox and use Effects to send our messages, the other is to create a custom version of StartApp that returns an additional value for things to send to the port in update. I have implemented both attempts as gists, here is the one with the Mailbox and once with a modified StartApp. For the rest of the blogpost I will refer to the first version since it works with the vanilla StartApp. Ok, let’s hook up the Mailbox:

portRequestEncryptionMailbox : Mailbox String
portRequestEncryptionMailbox =
  mailbox ""

port requestEncryption : Signal String
port requestEncryption =
  portRequestEncryptionMailbox.signal

This initalizes the Mailbox and fills in the hole we had before - the Signal we use as a port will just be the Signal of our new Mailbox. But how do we actually send anything to this Mailbox? By creating an Effect, like so:

sendStringToBeEncrypted : String -> Effects Action
sendStringToBeEncrypted clearText =
  Signal.send portRequestEncryptionMailbox.address clearText
  |> Effects.task
  |> Effects.map (\_ -> Noop)

-- and this is our update function that now returns a tuple of (Model, Effect):
update : Action -> Model -> (Model, Effects Action)
update action model =
  case action of
    TextChanged text ->
      ( { model | clearText = text }
      , sendStringToBeEncrypted text -- create the Effect here
      )
    -- other cases ...

The last line in sendStringToBeEncrypted may be a bit confusing - what are we mapping there? Let’s take a look at the type of Mailbox.send:

send : Address a -> a -> Task x ()

This means that the success type of the task we get back from send is () (aka Unit) which acts a bit similar to void in C like languages, i.e. it represents “no actual value”. Let me desugar sendStringToBeEncrypted so it is clearer what types we are working with:

sendStringToBeEncrypted : String -> Effects Action
sendStringToBeEncrypted clearText =
  sendTask : Task x ()
  sendTask = Signal.send portRequestEncryptionMailbox.address clearText

  effectOfUnit : Effects ()
  effectOfUnit = Effects.task sendTask

  effectOfAction : Effects Action
  effectOfAction = Effects.map (\_ -> Noop)

(Note that instead of first converting to Effects () and then mapping that I could also have mapped the Task x () to Task x Action and then converted the Task to Effects). This may still seem a bit weird but it may help to realize that in this particular case of sending a message to a mailbox we are explicitly not interested in an actual Action value. We are only interested in performing the side effect of sending the message and it will not have a reasonable “payload” that should be routed through update. But because of how Effects are typed in StartApp, we do need to have an Effects Action in the end and so we introduce a Noop value that explicitly does nothing when it is processed in update.

At this point it is important to remember that StartApp.start returns a record that contains a tasks field and this has to be wired to an outgoing port - if you don’t do this, the Tasks you create via Effects will never be executed by the runtime:

app =
  StartApp.start
    { init = init
    , update = update
    , view = view
    , inputs = []
    }

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

Ok great, this is the outgoing part of the ports - how about handling stuff that comes into our Elm program?

Incoming Ports

Let’s start on the Javascript side. We will define an incoming port called encryptionCompleted on the Elm side, and here we see how to send messages to it from JS. (Note that this example simplifies the logic a little and immediately after receiving a message from the outgoing port it sends an encrypted value back to Elm via the incoming port - in practice encryptString would probably call an API that returns a promise and only when this is fullfilled call send to send a value back to Elm)

var div = document.getElementById('root');
var myapp = Elm.embed(Elm.MyApp, div, {encryptionCompleted : ""});
myapp.ports.requestEncryption.subscribe(encryptString);

function encryptString(message) {
    encryptedMessage = "Encypted: " + message; // actually encypting the message is ommited
    myapp.ports.encryptionCompleted.send(encryptedMessage);
}

Note that I not only had to modify encryptString but also pass in an initial value at the time when we initialize Elm with the call to Elm.embed. The third parameter takes the initial value of every incoming Signal we define on the Elm side - it is required because Signals in Elm always need an initial value. Let’s add this incoming port on the Elm side to complete the example:

port encryptionCompleted : Signal String

Note that this time the port we have defined has no “implementation” in Elm. That is because, viewed from Elm, this is just an external input - a Signal we can use to trigger behaviour in our app. But how can we do that? How can we wire up this Signal into our StartApp.start call?

In the last post, when we switched from StartApp.Simple to StartApp, I mentioned inputs. inputs is a List of (Signal of Action), i.e. Signals that fire Actions that will be combined with the Signal of the main mailbox that is administered by StartApp. So this is exactly what we want to have for this little program so it can react to the Signal that represents the port - the only thing that is missing is that we have defined encryptionCompleted as a Signal of String and we need a Signal of Action for inputs. Sounds like we need a map again:

encryptedString : Signal Action
encryptedString =
  Signal.map EncryptedValueReceived encryptionCompleted

app =
  StartApp.start
    { init = (init, Effects.none)
    , view = view
    , update = update
    , inputs = [ encryptedString ]  
    }

And voilà! We have a Signal of Actions that we can put into the inputs part of start.

So just to recap, let’s go through the example again: Whenever the user enters text we process the TextChanged value in our update function and not only update the model but also create a new Effects. This is then returned by update and, because we wired the tasks part returned by start to an outgoing port, it is handed to the runtime. This leads, on the native JS side, to a call of encryptString (because this is the function we registered with subscribe). In it we pretended to do some encryption and then sent the value back with encryptionCompleted.send (again, you can send values at any time, it only happens in our example that we send one value back for every value we receive on the JS side). This send call leads the encryptionCompleted Signal to fire, with the string value we sent from the JS side. This is than mapped into an Action value, namely EncryptedValueReceived, and because this is hooked up to the inputs part of StartApp it triggers the same chain through update as any other events. In update we then handle processing of this EncryptedValueReceived value and the whole exercise is complete.

Thanks for reading through this long post! I hope it was useful and I appreciate any feedback you might have!