# Testing programs with ports

This guide requires that you're already familiar with testing programs that use Cmds. Specifically, you should understand how to set up your program for testing by creating a custom Effect type and implementing a simulateEffects function. If you aren't yet familiar with doing that, you should follow the "testing programs that use Cmds" guide first and return to this guide afterwards.

# Introducing the example program

The rest of this section will be working with an example program that interacts with the fictional "GudWerdz" service for checking the grammar and writing style of a text input.

The fictional "GudWerdz" service only provides a JavaScript SDK, so our app needs to interact with it using ports (opens new window).

architecture diagram described in the preceding paragraph

# Goal: the ideal test

The test we want to write represents the following scenario:

  1. The user fills in the text area with some text.
  2. The user clicks the "Check" button.
  3. The app sends the text to check out through the checkGrammar port, which will call the GudWerdz API.
  4. The GudWerdz API returns a list of problems with the text, which get sent to the checkGrammarResults port.
  5. The app receives the results and displays them to the user.

Here's what that test will look like in code:


import Expect
import Json.Decode
import Json.Encode
import ProgramTest
import Test exposing (test)

test "checking grammar" <|
    \() ->
            |> ProgramTest.fillIn "main"
                "Enter text to check"
                "The youngest man the boat."
            |> ProgramTest.clickButton "Check"
            |> ProgramTest.ensureOutgoingPortValues
                (Expect.equal [ "The youngest man the boat." ])
            |> ProgramTest.simulateIncomingPort
                (Json.Encode.list Json.Encode.string
                    [ "Garden-path sentences can confuse the reader." ]
            |> ProgramTest.expectViewHas
                [ text "Garden-path sentences can confuse the reader." ]

After entering some text and clicking the "Check" button, the test uses ensureOutgoingPortValues to assert that the entered text has been sent out on the "checkGrammar" port. Then simulateIncomingPort is used to simulate a response coming in on the "grammarCheckResults" port.

However, before this test can pass we'll have to do some setup work so that the test can simulate the incoming and outgoing ports.

# Simulating outgoing ports

We'll assume that we already have our test set up with ProgramTest.withSimulatedEffects (if you're not familiar with that, you should first follow the "testing programs that use Cmds" guide).

To add simulation of our port effect, we'll make use of SimulatedEffect.Ports.send in our simulateEffects function:

import Json.Encode
import SimulatedEffect.Ports

simulateEffects : Main.Effect -> ProgramTest.SimulatedEffect Main.Msg
simulateEffects effect =
    case effect of
        Main.CheckGrammarEffect text ->
            SimulatedEffect.Ports.send "checkGrammar" (Json.Encode.string text)

# Simulating incoming ports

Similar to how we provided a simulateEffects function for testing our Cmds, we'll need to provide a simulateSubscriptions function for testing our Subs. This will parallel the subscriptions function in our main program, but will return ProgramTest.SimulatedSub instead of the normal Sub. (This is necessary because, as with Cmds, Elm currently provides no way to inspect the value of a Sub.)

For reference, here's our program's subscriptions function:

port grammarCheckResults : (List String -> msg) -> Sub msg

subscriptions : Model -> Sub Msg
subscriptions _ =
    grammarCheckResults GrammarCheckResults

And here's the corresponding simulateSubscriptions function we'll write for our test setup:

simulateSubscriptions : Main.Model -> ProgramTest.SimulatedSub Main.Msg
simulateSubscriptions _ =
    SimulatedEffect.Ports.subscribe "grammarCheckResults"
        (Json.Decode.list Json.Decode.string)

Finally, we configure our ProgramTest with the simulateSubscriptions function before starting it:


start : ProgramTest Main.Msg Main.Model Main.Effect
start =
        { init = Main.init
        , update = Main.update
        , view = Main.view
        |> ProgramTest.withSimulatedEffects simulateEffects
        |> ProgramTest.withSimulatedSubscriptions simulateSub
        |> ProgramTest.start ()

# Try it out

You can play around with the example here: https://ellie-app.com/6wskFByvrPta1 (opens new window) Try editing the tests or the implementation to make the tests fail and see what kind of failure messages you get.