Basic debugging in Elm

This blog purely focus on very basic debugging. No time travel or anything fancy. Simple debugging features.

We will start with our baisc code which takes first name and last name in two different input field and when user pressed ‘FullName’ button, it displays the firstname + lastname in Full Name: label.


The program I started out is here in the Gist. With that code, when you run the application and enter the first name and last name like the following

and press FullName, you get the following result


The last name is missing and the first name is added twice. (It is easy to fix but for the sake of debugging, we will continue as if we do not know the reasons).  So how do we go about debugging in the Elm?

For starters, when you run the elm in elm-reactor mode, the application runs in debug mode. When the application running in the browers, you will see small box at the bottom right hand side of the screen something like the following


If you click to open the window you will see two sections

One on the left, shows all the events that triggered. The one on the right side shows the current state of the model.


This will be great means to see all the events that get fired and also for each event how the model changes.

The second approach is the age old logging. Elm provides a mean to log messages to the browser console. We will start with the first problem, that is, the result of first name and last name shows first name twice.

We have following code for Add message

Add ->
           { model
               | fullName = model.firstName ++ model.firstName
               , firstName = “”
               , lastName = “”

even though it is very clear that, we are adding firstName field twice, for the sake of showing logging, we will add logs. To add logs in Elm, you need to import Debug package, which is already part of the Core package you have installed.

1. Import Debug

2. Add log messages to see what is the value of the two fields we are adding

Adding log is pretty simple and very original. Before adding the log the Add event function was

Add ->

{ model

| fullName = model.firstName ++ model.firstName

, firstName = “”

, lastName = “”


To log you do not need to create a seperate line statement. You can do it inline like the following

Add ->
     { model
         | fullName = (log “first name : ” model.firstName) ++ (log “last name : ” model.firstName)
         , firstName = “”
         , lastName = “”

Log function take two parameters, first one is string message and second parameter can be anything. Here is an interesting thing, in elm when you have log statement like this, log function logs the message but the underlying function (the original) executes normally. So in reality, the before and after program statement is same.

Open the developer console and execute the code and you will see the first and last name both are first name. So looking at the code we know the insted of adding the last name we were adding the first name.


Change the first name field to last name and run the code with the log and now we get different message. Instead of repeating the first name twice, now it displays only the first name and not the last name.


You can find the code with logs here in this Gist. Now let’s look why are we not getting the last name. We are going to use, time trave/history tab to see what is happening.  When a user enters a value in the input, for every keystroke onInput event fired. So let’s start there. We need to check and see if appropriate action event fired.

Based on the update event,

type Msg
     = Add
     | Clear
     | InputFN String
     | InputLN String

We expect InputLN event fired. Let’s open the tab and monitor the events.

First I typed ‘unni’ and you can see blow, InputFN fired for each keystroke and when I clicked FullName, you can see the Add event fired as well.


With this much information first thing we need to do is to check, do we have the view wired up properly to fire the events on data entry.

, input [ onInput InputFN ] []
, input [] []

The error is obvious, for the second input box, we do not have an onInput event.

Adding the onInput and running the code gives the expected answer.


This is the first stab at the debugging in Elm. You can find the final version of code here in this Gist.


Working with Input Fields in Elm

Continuing on the ‘SayMyName/Say Hello’ example, we will examine the input tag to understand the events and state management. Like other examples, let’s look at the final output first

We have added a new button ‘Clear’. When someone clicks clear button, it will clears the value in the input tag. 

Like other examples, we will start from the Model. There is no difference between the previous example model and this one.

type alias Model =
     { name : String
     , inputValue : String

initData : Model
initData =
     { name = “”
     , inputValue = “”

Next Update. Like we discussed in the previous blog, update has two parts, one to define a message associated with an action and second, execution or acting on action.

Defining the action:

type Msg
     = SayMyName
     | InputData String
     | Clear

We added the last message ‘Clear’ to our valid actions. Next we need to act on it. If and when someone presses the clear button, what we need is to clear the value of the input box. ‘inputValue’ holds the current value of the input box, so in our action execution, all we have to do is to set it to empty string. Something like the following

update : Msg -> Model -> Model
update msg model =
     case msg of
         SayMyName ->
             { model | name = model.inputValue, inputValue = “” }

        InputData val ->
             { model | inputValue = val }

        Clear ->
             { model | inputValue = “” }

The last two lines were added to clear the input value.

Finally view, in view we have to do two things;

  1. Define new Clear button and onClick we send out ‘Clear’ event. That is achieved through following definition in view

, button [ onClick Clear ] [ text “Clear” ]

2. When a user clicks the clear button our update function clears the value of inputValue in the model. To reflect the current value of the model, we have to set the value of the input box to model’s inputValue like the following

, input [ onInput InputData, value model.inputValue ] []

You can find full code here at this Gist.

Unit testing in Elm

Elm shines in help build applications which does not crash in production.  With static typing, functional nature and compile time verification the odds of breaking Elm application is low. That is been said, it does not give you pass not to write tests. I talked in length over time about TDD and why TDD is the best way to build applications. Here are two important things you need to know about TDD

  1. Help build highly maintainable code because the tests guarantee you will not break any other part of the application.
  2. TDD when done right, will allow you to write very little code to meet the expectation of the requirements. Less code you write less bugs you introduce.

Since everything in Elm is function, we can test everything.

To setup, we need to install elm-test package by running

elm package install elm-community/elm-test

This will install all the packages required for building unit tests. Once install, let’s initialize project structure for creating unit tests by running following in application root directory

elm-test init

This will create a test folder and create a simple test with a ‘todo’ and install associated project dependencies. So before you do anything, run


in the root directory and it should run the default tests and come back with following message

elm-test 0.18.8

Running 1 test. To reproduce these results, run: elm-test –fuzz 100 –seed 5741

TEST RUN INCOMPLETE because there is 1 TODO remaining

If you get the above message then all setup and ready to start write testing. If you have problem setting up the test environment, send me a message.

I love to do my tests using BDD model and fortunately, there is a package for that which makes it very readable. When ‘elm-test init’ ran it created a sub directory called test to write all your tests. Switch to the test directory and install the BDD package

elm package install rogeriochaves/elm-test-bdd-style

Let’s get our feet wet with nothing fancy but just do normal assertions to see how they all connected. Create a a file in test subdirectory of the project root directory. Create an elm file, I called it ‘SayMyNameTester.elm”

Let’s start importing the required packages for testing

import Test exposing (..)
import Expect exposing (..)
import ElmTestBDDStyle exposing (..)

basictest : Test
basictest =
     describe “Tests Basic Assertions”
         [ test “1. can test equal traditionally”
             (\() -> Expect.equal “test” “test”)
         , test “2. can test equal traditionally with infix” <|
             \() -> Expect.equal “test” “test”
         , it “3. can test equal with bdd with infix” <|
             expect “test” to equal “test”
         , it “4. can test equal with bdd more readable”
             (expect “test” to equal “test”)

The above test is nothing but doing a simple assertion and “test” equal to “test” which results in true. But it was written in four different ways to show different ways of writing it and pick the one you are comfortable with and go with it.

When you run Elm-Test at the project root directory, it will executes all the files in the test directory with Test/Suite function definitions. In our code we have just one function definition with Test signature, which is basictest.  So running the Elm-Test should result something like the following


Let’s walk through each of the tests to understand what is the different

Test 1:

test “1. can test equal traditionally”
             (\() -> Expect.equal “test” “test”)

For starters, test function, takes a string and an Assertions and return a Test. The first parameter is “1. can test equal traditionally” a string. The second parameter is the Assertions “(\() -> Expect.equal “test” “test”)” which returns a Bool. For the people who are coming from traditional programming language,

\() –>

represents an anonymous function. Also the second parameter is enclosed in () so that it gets executed to return Test result.

Test 2:

This is exactly same as Test 1 except we replaced the () with an infix notation backward pipe operator <| Backward pipe and forward pipe operator (|>) or interesting operators of the functional programming, which got its roots from Unix world. You can read more about it here.  For me this gives more readability than using (). When you compose lot of operations, when using () might make it bit hard to read. So I prefer to use pipe operator

Test 3 & 4:

Test 3 & 4 are similar variations of Test 1 & 2 except, 3 & 4 is written in BDD style. Compare to 1 & 2, 3 & 4 are readable.

My recommendation to use the BDD style option 3 for more readability. Especially when you are building large code base, help the people who are coming after to do their job easier by writing readable tests. They will thank you for doing it.

I was going to write how to write a unit test for our project but I found a great video of Andy Balaam, who walks through the whole unit testing. It is worth checking out.


Continuing the read code – Finally the view

We are building a very simple application which does not do anything fancy so that we can focus on the fundamentals of Elm. In the past two blogs we looked at

  • Model
    • We talked about type alias to keep the code readable by defining the alias for the core data structure so that you do not need to define it every time when one need to define the function definition
    • We also need to define the initial state of the application as part of the model definition. The main reason we need this to make sure on page load, the view is rendered through the initial state of the application
  • Update
    • Compared to other programming model, we need to define all the actions that are going to happen in a given application by defining the message types by using Elm specific type (union type or traditional enumeration type)
    • We need to create a function which will then act on a given action by changing the current state to new state, which will then trigger view rendering by taking the current state of the application and build the modified DOM elements

Basically Model defined the initial state and data structure while Update create the current state of the application. With that what is left is to render the view.

Traditionally most of the frameworks uses HTML or a template to build the view. In Elm, a true functional programming language even rendering HTML view done through functional model. In Elm, every html tag is a function. Each of these functions takes two list of arguments. First one is all attributes and second is list of all its children.

Let’s look at the code which will render our HTML page and then we can deconstruct it

view: Model –> Html Msg

view model =

    div []


        h3 [ ] [ text “Your name is: “ ++]

        , input  [ onInput Change

                      , value model.inputdata ] []

       , button [ onClick SayMyName ]

        [ text “Say My Name”]


Let’s go through this code.

view: Model –> Html Msg is the function definition of the view function. View function takes model as input and generate Html and Html which could create messages

The first function call in the view is div function. As stated before, first array of list represent the attributes and second array is the collection of its children.


As specified in the diagram above, the first array is empty, but the second array, has three children


So far we have defined the model, update and view. As mentioned in the theory part of the Elm, we need an entry point and plumping needed to connect all these three components. That entry point is main as defined below

main =
     beginnerProgram { model = initData, view = view, update = update }

For getting started, we will leverage a function called beginnerProgram function and pass in the initial state of the model, the view and update function. The beginnerProgram will trigger the following on load

  1. First initial state of the mode created by running initData
  2. View function will take initData and render the HTML DOM elements
  3. As the user enters data in the input field, Change event will be triggered.
  4. Update function is called every time appropriate events are triggered
  5. Update function will be called with Change function and it will pass in the current value of the input text
  6. Update function execute Change event, which does nothing put populate inputdata with the input box value
  7. When the user clicks SayMyName button, view will trigger SayMyName event
  8. Update function called again with SayMyName event
  9. Update function executes SayMyName code block, which creates new model with name being populated with inputdata and inputdata set to blank, which represents the current state of the application
  10. When ever model changes, view function get called with current state of the application and repeat step (2) and forward again

You can find the full code here at the Gist.

Continuing the real code – Let’s talk about update

We are working on building a real code after series of theory blogs. We are building a very simple application which allows a user to enter a name (any string) in the input box and when the user clicks the ‘Say My Name’ button it will display the name in ‘Your name is:’ label.


In the last blog, we looked the model component. In this blog we will look at the update component.

Think of update component as a controller in a normal MVC pattern. This part of the code listens for event changes and update the model accordingly. Once the model updated the view will be rendered again to show the event’s reaction.

Like I said previously, update component reacts to the events. Like model, update also has two components

  1. Define all the events that we are interested in listening
  2. Write code to perform tasks when an event happens

Based on the end state of the application, what are all the possible events that can occur in the application. The obvious first event will be some one clicking the button ‘Say My Name” and let’s name the event  ‘SayMyName’.  The second one will be, when ever a user enters something in the input box, data change event for the input box will be triggered. Let’s name the event as ‘InputChange’ event. That is about it.

Next question, how do we go about defining these events? In Elm you define these events with type like the following. We identified the two events and we just create a type with the two events.

type Msg = SayMyName | InputChange String

You might have question how do you define this named event and where do we define them? We will look at them when we discuss the View component in detail. For now, when someone clicks the “Say My Name” button, some how the ‘SayMyName’ message will be triggered in update section.

InputChange event definition though is little different. When someone click the button, all the framework does it to trigger an event. On the otherhand, when someone changes a value in the input field, a change event get triggered with the changed value. That is why, InputChange event defined with one String parameter.

Now that we defined the events, let’s look at what are we going to do when these events occur?

update: Msg –> Model –> Model

because Elm is static type, it is good to define the update function first.Based on the signature, update function takes the message (Msg) as first parameter, which also takes Model (current state of the application) as second parameter and at the end of the update function, it returns modified state of the application model (Model).

In the last blog we looked at the Model. In that, we identified the two required fields. One to display the name (name) and another one to hold the input data in the input field(inputdata). With that knowledge, when a user clicks the ‘Say My Name’ button, we need to do the following

  • Copy inputdata to name field
  • Reset the inputdata field to be empty (“”)

How do we do it in functional Elm? We need to take the incoming model and modify name and inputdata. The way you modify records in Elm as follows

{model | name = model.inputdata, inputdata = “”}

following the code model above, when there is a change to the input field, we need take the new value and put it in the inputdata field.

{ model | inputdata = val}

where the val is incoming value on data change. It will make sense in a bit when we look at the whole update function.

update msg model =

    case msg of

         SayMyName –>

               {model | name = model.inputdata, inputdata = “”}

         InputChange  val –>

                {model | inputdata = val }

Update function take two parameters, first is the event that triggers the update function and also the current state of the data (as model)

In Elm, we check what event was triggered by doing something similar to switch statements in other languages. If you look at the code, there are no return statements. In functional programming there are no explicit return statements instead the last statement is treated as return statement. In our update function definition, update function is supposed to return a model. If you look at the each of the case statement, both has just one line statement and both return new instance of the model with new data.

Next we will talk about view

First taste at real code – Let’s talk about Model

Now that we know what makes up Elm and also have your development environment setup, let’s start doing some coding and learn along the way.

For our first exercise we are going to build a very simple application like the following


This application will say what ever we type in the input box. On start there is no data so it is not showing anything. If I were to type my name ‘Unni’


and hit the ‘Say My Name’ button, it will be something like the following


If you notice, right after you click the ‘Say My Name’ button, the name appeared in the first line but the input field cleared.

Let’s get started.  When we build any Elm application, you need define the Model, Update and View. One best place to start is Model(or Data).

Before we go into our example code, I want to talk about model quick. Model is nothing but your application’s data structure if you will.In Elm Model has two parts,

  • Declaration of the Model
  • The startup/initialized data of the model

If the model is very simple like single data type in it, we could skip the model declaration altogether but it is best practice to always define the Model. You can define the model with ‘type’ definition. But it is always recommended to use ‘type alias’ to define the model, which will make the code much more readable. We will get to that later, for now let us assume, if you are going to define a model, always use ‘type alias’.

Now that is out of the way, let us talk about what should be our model based on the application we are building. By looking at the screen, you can see we need two data fields.

  1. A field is hold the value of the name till someone presses the ‘Say My Name’ button, let’s call that as ‘inputdata’
  2. On clicking the ‘Say My Name’ button, we need to display the name from the input field (inputdata) also at the same time we need to clear the inputdata. So we need another variable to hold display name. Let’s call that ‘name’

Now that we know we need two variables, what type are they? In this simple case both are strings. So let’s create the model data structure first

type alias Model =
     { name : String
     , inputdata : String

If you remember the anotomy of Elm application blog, I mentioned, the view take a model to display  the view. So we need to pass start up date to view. Now that we know the model, let’s create the startup data.

initData : Model
initData =
     { name = “”
     , inputdata = “”

Elm is static typed language so any thing you define, you need to declare its definition. Along the same line, when we define a variable, we need to define what type it is. So the first line

initData : Model

declared the initData is of type Model. Now just imagine, if you did not define the type alias it will look something like this

initData : { name : String, inputData : String }

Obviously using type alias make the code more readable. Next four lines instantiate/assign the initial value of the initData. Both the name to display and input data are empty strings.

Hope you are with me so far. I want to take a moment to correct few things I mentioned earlier in this blog. Elm is a functional programming language, that means there is no mutation. Everything is immutable. In the above code I said name and inputdata as variable. They are not really variables, they are constants in a way. If you were to change the value of either of the fields, Elm runtime will create a new instance of those fields.

Following the same model, the initData is a immutable initial startup data.  If everything is mutable, how do we hold state of an application? Hold the thought we will come back and address that later.

We will look at update component next.

Setting up Elm development environment

Now that we know some of the core fundamentals and things that make up Elm, let’s start learning by coding. For that, we need to setup the development environment first. It is simple and easy.

I was going to write step by step instructions on how to setup Elm for Atom editor but found this great Git repo. This has all the information needed to setup. Few of things you need to know as you go through them

  • Where ever, it say ‘which’ replace with ‘where’ for Windows
  • If you are using Atom if for some reason something not working, restart Atom
  • linter-elm-make required elm-make.exe, if you are in Windows, make sure when you paste the location of the elm-make, put the whole path and the file name in double quotes like the following

“C:\Program Files (x86)\Elm Platform\0.18\bin\elm-make.exe”

  • When you start a new Elm file, the Elm language may not work, for that make sure you install elm-package. Atom will complain about missing json file. Execute the following in the command prompt

elm-package install

If you face any problem following the git repo, let me know I will try to help