Elm-lite, in javascript

Introduction

Elm is a fantastic programming language. Through functional programming and the Elm architecture, it facilitates a style of writing web applications, that allows for code that is modular, reliable, and easy to refactor.

If you are starting a new project that requires a fast interactive front-end and you are willing to learn a new language and ecosystem, I would strongly encourage you to choose Elm.

But if you don't have that flexibility, say you want to stay inside the javascript world and borrow some of the fantastic ideas behind Elm, this blog post is an attempt to show you how.

In this blog post, we'll build something similar to fantasy football, where there is a list of all available players and you can draft them into your fantasy team, but to do it for the US Congress and Senate. The application we'll be building is available here and the source code is available on GitHub.

The most important tool to achieve the architecture we are going for is functional programming. If you're unfamiliar with FP, I would point you towards this book as a wonderful introduction.

We will use a library Ramda which I find to be the swiss army knife of functional javascript. It builds on the ideas of utility libraries like Lodash by exposing methods that are "functional first".

Wiring the application

The essential wiring we'll use to try to emulate the Elm architecture is by leveraging a module called main-loop.

Main loop allows us to create a mechanism for taking the state of our application, represented in a javascript object, and run it through a render function that turns that state into a virtual dom node. We can the attach the result of that operation and append it on to the page. This will put the visual representation of the state on the page.

var mainLoop = require('main-loop');

var render = function(state) {
  // Coming up soon!
}

var initialState = {
  title: 'test'
};

var loop = mainLoop(initialState, render, vdom);

document.querySelector('#content').appendChild(loop.target);

Where the magic happens is that we can call the update method on loop at any point and pass it a new state. When we do that, the virtual-dom will take care of figuring out the which minimal updates need to be applied to the dom to make the new representation we need.

loop.update({
  title: 'new title'
});

Hyperscript Helpers

One of my favorite features of Elm is the ability to represent html in normal everyday functions. That allows you to run any normal operations (like map) to calculate your views. It also allows you to unit test your views just like any other function.

titleView state =
  h1 [] [ text state.title ]

Luckily there's a JS library called hyperscript-helpers that brings that functionality!

var titleView = function(state){
  h1('.title', state.title);
};

Rendering arbitrary lists

Initial State

Our initial state will have two properties, both arrays of objects, availableLegislators and selectedLegislators.

/* Initial state */
var initialState =  {
  selectedLegislators: [{
    firstName: 'Juan',
    lastName: 'Caicedo'
  }, {
    firstName: 'Carson',
    lastName: 'Banov'
  }],
  availableLegislators: [{
    firstName: 'Senator',
    lastName: 'One'
  }, {
    firstName: 'Congresswoman',
    lastName: 'Two'
  }]
};

This is the initial state that we will pass to the mainLoop function, along with the virtual-dom module and the rendering function we will be describing next.

Render functions

Let's start out with the main render function. This function only needs to create a main div and then it can delegate the rendering of each of the two arrays in the state.

var render = function (state) {
  return div('.container', [
    legislatorTableView('Your Team', state.selectedLegislators),
    legislatorTableView('Available', state.availableLegislators)
  ]);
};

Both of the children to this main div will be structurally identically, the only difference will be the title rendered above the table and the actual contents of the table. We can encapsulate these similarities by defining a new function legislatorTableView and passing the differences in as parameters.

var legislatorTableView = function (title, legislators) {
  return div('.col-xs-6', [
    h1(title),
    table('.table.table-striped', [
      tbody(
        R.map(legislatorView, legislators)
      )
    ])
  ]);
};

The legislatorTableView renders another div (adding some bootstrap styling to make it take up half of the screen), renders the title we passed in, initiates another table, and delegates the rendering of each row to another function legislatorView.

var legislatorView = function (legislator) {
  return tr('.container-fluid', [
    td('.col-xs-6', legislator.firstName),
    td('.col-xs-6', legislator.lastName)
  ]);
});

This function legislatorView is the lowest level of our rendering, so it just takes care of turning a single object into a row of a table with two cells.

With all those view functions, we now have a way of rendering our initial state into a page with two tables side by side, each with two rows.

Dealing with updates

We would now like to add functionality so that if you click on a legislator, it will move that legislator from one table to the other. hyperscript-helpers exposes a simple way to wire up onclick handlers, but that means that we need to be able to trigger an update from inside the rendering code.

main-loop gives us the .update function, but we need to first instantiate the loop before we can call it. Since we have to pass the render function in order to instantiate the loop, there's no way the render function can call loop.update directly.

Actions

In Elm, updating is done through an indirect, but wonderfully elegant mechanism. Views have access to an "address" to which they can pass an "action". An action is just a structure that has a type and some data associated with it. These actions all find their way eventually to an update function that knows how to calculate a new state given the old state and an action.

We can emulate this mechanism by first creating an action function to instantiate this data structure.

var action = function(type, data) {
  return {
    type: type,
    data: data
  };
};

Update events

Next we will need to wire up some unavoidable stateful code. This code will go in our "unsafe" section of the code, like instantiating the main loop.

We instantiate a new event emitter. Here we're doing it with Node's built-in EventEmitter, since we'll be using browserify to build a client-side bundle, but the same could be done with the DOM events or with jQuery's events.

var emitter = new EventEmitter();

The address function here just takes care of triggering an event of type 'update' and sending the action it gets along as data with the event. It's worth noting that the addresses used in the Elm architecture are actually part of a more complex update wiring which is therefore much more robust, but for these purposes, a simple address illustrates the idea.

function address(action) {
  emitter.emit('update', action);
};

Then we register an event listener on any update event. It will call update passing it the current state of loop and action to calculate a new state with.

emitter.on('update', function(action) {
  var newState = update(loop.state, action);
  loop.update(newState);
});

Changing state

update simply needs to be a function that checks the type of the action and uses its data to calculate a new state. This is not very complicated at all, simply some appending to the one list and removing from the other, depending on the direction. Though it would be possible to return these two properties as a new state, I prefer to use R.merge, which mirrors how Elm "modifies" records. It creates a new object from the first object and the properties of the second object.

var update = function (state, action) {
  var newSelected;
  var newAvailable;

  // fallback case
  var newState = state;

  if (action.type === 'Drop') {
    newSelected = R.reject(R.equals(action.data), state.selectedLegislators);
    newAvailable = R.append(action.data, state.availableLegislators);

    newState = R.merge(state, {
      selectedLegislators: newSelected,
      availableLegislators: newAvailable
    });
  } else if (action.type === 'Select') {
    newSelected = R.append(action.data, state.selectedLegislators);
    newAvailable = R.reject(R.equals(action.data), state.availableLegislators);

    newState = R.merge(state, {
      selectedLegislators: newSelected,
      availableLegislators: newAvailable
    });
  }
  return newState;
};

One great feature of this update mechanism is that it does not ever reference any state outside the function definition and it is therefore very easy to test!

Triggering updates from views

We can change the definition of render to now also take address as a parameter. We will also curry the whole function so that we will be able to specify address without needing to specify state (as that will be specified when main-loop calls that function). We will then pass the address function as well as the type of the action to send whenever a use clicks on a row to legislatorTableView.

var render = R.curry(function (address, state) {
  return div('.container', [
    legislatorSelectView(address, 'Drop', 'Your Team', state.selectedLegislators),
    legislatorSelectView(address, 'Select', 'Available', state.availableLegislators)
  ]);
});

This also means that we should change how we instantiate the main loop since we changed the definition of render.

var loop = mainLoop(initialState, render(address), vdom);

The definition of legislatorTableView will also change, but not by much. Now we will just add the two new arguments, and we will pass them to legislatorView. Note that we will also be currying legislatorView, so we can call it with address and type to partially apply those arguments.

var legislatorTableView = function (address, type, title, legislators) {
  return div('.col-xs-6', [
    h1(title),
    table('.table.table-striped', [
      tbody(
        R.map(legislatorView(address, type), legislators)
      )
    ])
  ]);
};

And now down at the legislatorView level, we will actually wire up a a call to address. hyperscript-helpers allows us to pass an object to any element we are instantiating and specify attributes that should have. We can specify an onclick handler and when it's called simply call address, passing it an action from our type and the current legislator. Then address will trigger an update event and our update function!

var legislatorView = R.curry(function (address, type, legislator) {
  return tr('.container-fluid', {
    onclick: function(ev) {
      address(action('Toggle', action(type, legislator)));
    }
  }, [
    td('.col-xs-6', legislator.firstName),
    td('.col-xs-6', legislator.lastName)
  ]);
});

Requesting data asynchronously

Tasks

Another fantastic feature of Elm is how it represents asynchronous actions as data. It does that through a data structure known as a Tasks. They are somewhat similar to promises, except that they are completely stateless and in order to execute a task you have to specify what to do if the task succeeds and what to do if it fails.

This design is extremely useful because it allows you to create the logic of your stateful asynchronous actions in stateless functions. This allows you to isolate effects of those operations, keeping the majority of code still easy to test and refactor.

var getJSON = function (url, params) {
  return new Task(function(reject, result) {
    $.getJSON(url, params, result).fail(reject);
  });

You can then save the task and execute in in a controlled environment when you are ready to deal with the results of it.

var myTask = getJSON('url', {apikey: 'key'});

myTask.fork(
  function(data) {
    // specify what to do if the task succeed
  },
  function(err) {
    // specify what to do with a json error
  });

Nested actions

To do this, first let's enable our update code and render code to deal with nested actions. This will allow us to have multiple top level actions, each divided into subactions.

First let's change legislatorView to send a Toggle action with two subactions, Drop and Select.

var legislatorView = R.curry(function (address, choice, legislator) {
  return tr('.container-fluid', {
    onclick: function(ev) {
      address(action('Toggle', action(choice, legislator)));
    }
  }, [
    td('.col-xs-6', legislator.firstName),
    td('.col-xs-6', legislator.lastName)
  ]);
});

And then let's change update to handle these nested actions.

var update = function (state, action) {
  var newSelected;
  var newAvailable;

  // fallback case
  var newState = state;

  if (action.type === 'Toggle') {
    if (action.data.type === 'Drop') {
      newSelected = R.reject(R.equals(action.data.data), state.selectedLegislators);
      newAvailable = R.append(action.data.data, state.availableLegislators);

      newState = R.merge(state, {
        selectedLegislators: newSelected,
        availableLegislators: newAvailable
      });
    } else if (action.data.type === 'Select') {
      newSelected = R.append(action.data.data, state.selectedLegislators);
      newAvailable = R.reject(R.equals(action.data.data), state.availableLegislators);

      newState = R.merge(state, {
        selectedLegislators: newSelected,
        availableLegislators: newAvailable
      });
    }
  }
  return newState;
};

Task-based JSON request

We discussed a task version of getJSON earlier, so now let's go ahead and make a function for calling it.

var fetchLegislators = function(address, jsonTask) {
  jsonTask.fork(
    function(error) {
      address(
        action(
          'PopulateAvailableLegislators',
          action(
            'Error',
            error
          )
        )
      );
    },
    function(response) {
      address(
        action(
          'PopulateAvailableLegislators',
          action(
            'Success',
            R.map(decodeLegislator, response.results)
          )
        )
      );
    }
  );
};

fetchLegislators is not really a stateless function: it has side-effects because it is executing an asynchronous task and it doesn't have a return value, but writing it will still allow us to keep our code more organized.

Note that although the function might seem complicated (25 lines of code!), it's really just saying "execute a json task, then send an action if it's successful and a different one if it fails". We pass the results of the response though a decoder function just because we would like the names of the properties to match that of our existing legislators (which is not what the api returns).

var decodeLegislator = function (legislator) {
  return {
    firstName: legislator.first_name,
    lastName: legislator.last_name
  };
};

Update based off ajax response

Let's now add another section to our update function to handle the action sent by the JSON request.

var update = function (state, action) {
  var newSelected;
  var newAvailable;

  // fallback case
  var newState = state;

  if (action.type === 'Toggle') {
    /* ... */
  } else if (action.type === 'PopulateAvailableLegislators') {
    if (action.data.type === 'Success') {
      newState = R.merge(state, {
        availableLegislators: action.data.data
      });
    } else if (action.data.type === 'Error') {
      console.log('Error', error);
    }
  }
  return newState;
};

Note that we are explicitly choosing not to do anything with the error, but it flows through the application just like other actions and we could instead attach it to the state and use it to render an error message for the user.

Triggering the JSON request

Now we can add a few more lines to the stateful section of our application to trigger the request.

var dataUrl = ''; // url to request data from'
var dataParams = {}; // api key and other query params
var jsonTask = getJSON(dataUrl, dataParams);
fetchLegislators(address, jsonTask);

We instantiate a task to make the json request and then we execute it by passing it to fetchLegislators.

Conclusion

Elm is a fantastic programming language, and the Elm architecture is a fantastic way to structure web applications. If it's not possible for you to start using Elm for your application, it's possible to get some benefits from copying some ideas from them into your javascript code.

If you like the ideas in this blog post, you should consider looking at olmo which is a very thorough look at porting the Elm architecture to javascript. And if you would like to build a full web application in this style, I would encourage you to look at Redux, which is a Flux implementation which borrows heavily from the Elm architecture.