Yesod, AngularJS and Fay

October 30, 2012

GravatarBy Michael Snoyman

It's been a while since I discussed Yesod's approach to client-side programming. I haven't been quiet due to a lack of interest. On the contrary, I've been playing around with a lot of different approaches, and discussing things with a number of people as well. Additionally, there have been some very exciting changes in the Haskell Javascript space.

Back when we discussed this in April, I demonstrated a combinator-based approach for generating Javascript from our Haskell code in a type-safe manner. After playing around with this a bit, I decided it was too inconvenient to be a focus of efforts. I've kept that code, together with some other attempts I've made, in a yesod-js repo on Github. If you're interested in the process I went through to come to the ideas I'm stating in this blog post, you might want to look there. Otherwise, the remainder of this post will be talking about future plans exclusively.

So the big question is: what do we want from a client-side solution? For me, it boils down to:

  1. A better language than Javascript. Ideally, I want the same level of static type checking in my client-side code as my server-side code. Having a better syntax than Javascript is a nice bonus, but not the main point for me.

  2. Tight server/client integration. I'd like automatic serialization of data between server and client. Or let's take it a step further: it would be great if it's almost transparent to the client-side code if we're calling a function locally or remotely.

    By "almost transparent", I mean we should still keep asynchronous calls asynchronous. In an ideal world, we could have a powerful multithreaded runtime like GHC provides. But dealing with explicit callbacks is acceptable IMO.

  3. We need to solve the big problems in client-side development as well. In other words: even if we could use GHC on the client side and compile any arbitrary piece of Haskell, we'd still have problems to solve: the DOM API is a mess, creating interactive UIs requires a lot of work, etc.

  4. We should not reinvent the wheel; we can use existing Javascript libraries when they're available. This might sound obvious, but I don't think it is (I'll give a concrete example later). There is a huge amount of effort going on into making client-side development better, and by simply barricading ourselves in our Haskell bubble, we'll be missing out on a lot of improvements out there.

    This is actually a more general Yesod philosophy: we should stick to tried-and-true techniques as much as possible, and simply make them better by adding in Haskell's strengths. An additional benefit of this is the reduced learning curve for developers coming from another language. (Or, conversely, the fact that you can reuse your Yesod skills when working with other languages.)


We can solve all four of these issues with a two-pronged approach. Starting with issues (3) and (4): we piggy-back on an existing, well-designed library in the Javascript world to solve the big problems. After some research and a lot of recommendations, I mocked up a demo using AngularJS. I have to say, I'm quite impressed. I won't by any means claim to be an expert, and I'm certainly not completely sold that it's the One True Path, but it does solve a lot of problems in an elegant manner.

You can see my sample code on Github. For this demo, I wrote the Javascript code as simple Javascript. One important trick I used here is providing a set of commands. For example:

cmdGetPeople <- addCommand $ \() -> do
    people' <- getYesod >>= liftIO . readIORef . ipeople
    return $ map (\(pid, Person name _) -> PersonSummary pid name) $ Map.toList people'

Each command takes a single JSON value as input, and returns a single JSON value as output. This is an important simplification over the standard approach of passing separate parameters: we can more clearly define our API, as will become important when coming to type safety. Calling this from Javascript is simple:

function($scope, $http) {
    $"#{cmdGetPeople}", []).success(function(data) {
        $scope.people = data;

Note: If you don't have experience with AngularJS, some of the code in here may not make much sense. Don't worry too much about that for now, I'm hoping to give more information about Angular in future blog posts, as the Yesod solutions for interacting with it mature.

The addCtrl Template Haskell function automatically includes the people.hamlet and people.julius files. cmdGetPeople is an automatically generated unique textual identifier given for the command defined.

Note, however, that there is no type safety in this approach.

I would classify AngularJS as giving us some level of reactive programming. We update variables, and the views update themselves automatically. It doesn't have all the power of a proper FRP solution (like reactive-banana or netwires), and it uses a bit of a hack- efficient dirty checking- to get it working. So for a long time I was opposed to using Angular: why use a hacky, suboptimal solution to a problem when we have a beautiful solution just waiting to be refined?

And the answer is simple: Angular is ready to be used now. I still believe that FRP will give us a better result in the end, and I hope the Haskell community continues to develop FRP solutions, and bring them into the client-side realm. But in the meanwhile, using Angular gives us a huge amount of the benefits we're looking for, while staying with mainstream web development.


The problem with this demo is that we're still programming everything in horrible, ugly, unsafe Javascript. I originally thought to overcome this with a combinator approach, but as I mentioned I think combinators will be too awkward overall.

After playing around with some alternatives, I tried out Fay, and I'm very impressed. It doesn't provide quite the full Haskell experience, but for most websites I think it's offering the right trade-off. The generated code, while not idiomatic Javascript, is still close enough to the source to be recognizable.

One huge selling point of Fay is its Foreign Function Interface (FFI). As an example, in Fay the alert function can be called via:

alert :: String -> Fay ()
alert = ffi "window.alert(%1)"

This simplicity makes it trivial to interact with existing libraries, such as jQuery. And as a result, I think Fay can be used as a drop-in replacement for arbitrary Javascript code. To test out that theory, I started converting a small project I've been writing to use Fay.

(For those of you wondering: this site is just a tool I built for my wife to sort our family photos. It's never seen the light of day before.)

You can view the commit in question to see exactly what I did. The goal was to replace one Julius file (incoming.julius) with a Fay file (Incoming.hs). Just to give a small test of the advantages of such a switch, compare this AJAX Javascript call:

    url: "@{AddPostR}",
    data: {
        date: $(this).parent().children("input[type=date]").val(),
        slug: $(this).parent().children("input[type=text]").val()
    dataType: "json",
    success: function() {
    error: function () {
        alert("Some error occurred");
    type: 'POST'

With this Fay type-safe callback:

t <- eventSource e
date <- parent t >>= childrenMatching "input[type=date]" >>= getVal
slug <- parent t >>= childrenMatching "input[type=text]" >>= getVal
call (AddPost date slug) $ const reload

Build Process

One problem people face is how to integrate Fay into your project's build process. Fay files are valid Haskell, and should be compiled with GHC to type-check them. Afterwards, they can be compiled by Fay into Javascript. However, I've gotten spoiled by Julius, and wanted to have automatic code reloading during development.

To address this, I created two Template Haskell functions. fayFileProd compiles with GHC, erroring out if the compile fails. Then, if that's successful, I use the Fay library to compile to Javascript. If that succeeds, I return a Javascript value, the same as Julius would have produced.

The other function is fayFileReload. Instead of compiling to Javascript at compile time, it performs the compilation at runtime, thereby automatically reloading any changes. The Fay compiler itself runs incredibly quickly, so there's no noticeable lag. And if you have errors in your Fay file, they show up in the browser:

Fay error message

Then there's a helper function called fayFile which will use fayFileProd during production builds and fayFileReload when using yesod devel. This won't catch type errors during development builds, but for me that's an acceptable trade-off: we get very fast response times, and our production builds are guaranteed to be type-safe. If you really want, you can manually run ghc to type-check your code.

I used fayFile just like I'd use widgetFile. No modifications to the build system necessary.

File structure

If you looked at the code above, you might have noticed some special folders. What I've set up is to subfolders: fay-shared is for modules used by both client- and server-side code (see the next section about this). The fay folder is for client-side-only code. My plan is to modify the yesod devel file checker to ignore file changes in the fay folder, thereby avoiding code reload when you're just playing around with client-side code.


In the code I gave for Angular above, I mentioned the concept of commands. It turns out that Chris came up with almost exactly the same approach for getting Fay to interact with the server. I've adopted his approach almost verbatim in this example.

In fay-shared, there's a single module which defines a Command datatype. This represents all of the commands that can be sent to the server, along with the results of such calls. In the Handler.Command module, I handle these requests on the server. The handle function is general-purpose, and could be put into a yesod-fay package. In handleCommand, I've written handler functions for each command. The type system ensures that the response we send back is the one the client is expecting.

Calling a command from the client is even easier. For example:

call (AddPost date slug) $ const reload

What this says is "Call the AddPost command with date and slug parameters, and when the call returns, I want you to ignore the response value and reload the page."

If you compare the original Javascript to the new Fay code, they're basically the same structure. Both are using jQuery and registering callbacks for events. Both are making AJAX calls to the server. But personally, I much prefer the Fay code: it's easier to read (as a Haskeller), and the AJAX calls are completely type-safe.

Why not ghcjs?

I played around with ghcjs. It's a truly amazing project: it allows you to compile virtually any GHC-understood code to Javascript. However, there are a few reasons I've decided to go with Fay for the moment:

  1. There's a difficult installation process for ghcjs, involving a modified version of GHC. Over time, I believe that this will be simplified as patches get merged upstream, but for many users it would be too difficult a bar to overcome.

  2. One of Fay's great features is its dead-simple FFI. By contrast, ghcjs has the traditional FFI, which makes more difficult to interact with Javascript libraries.

  3. Currently, the generated code won't run on Internet Explorer. I know there's work in the pipeline to address this, so longterm it shouldn't be a problem.

So if you read between the lines here, what I'm really saying is, "ghcjs isn't the right solution today, but it might be soon." It's a truly amazing effort, and I'm looking forward to where it heads. I think there's a lot of room for Fay and ghcjs to complement each other: Fay being a simpler tool for simple tasks, and ghcjs being the "big guns" when you need more power.

And Elm?

Another solution in this space which is developing nicely is Elm. It provides a client-side FRP solution, and with elm-yesod there's already the ability to integrate nicely with Yesod.

However, as much as Elm is influenced by Haskell, it's still a separate language. This may not be a problem for many cases, and if you think Elm would be the right fit, you should definitely give it a shot.

Next steps

I sketched out four problems I'm hoping to solve. As it stands, I think Fay solves the first two, and Angular solves the second two. So the next obvious step would be to integrate these two approaches. Initially, this looks pretty simple: using Angular required writing some Javascript code. Now we'll just write Fay code.

Fay is still a very young project, and therefore there are still some obstacles to overcome. In the process of writing this code, I came up with the following observations:

  • We really need to have some package management system for Fay. I'm not too concerned with exactly what this would look like, but it's important to be able to share common bindings like jQuery.

  • When sharing datatypes between the server and client, I'm forced to use String instead of Text (there are likely a few other examples of this too). It would be nice if Fay could automatically treat Text as a synonym for String, while for the server-side code use Data.Text.Text.

  • Chris's example code has a getThis function for retrieving the this object in Javascript. Unfortunately, due to how function calls work in Fay, this is almost meaningless. In particular, this does not point to the clicked DOM element when using a jQuery event handler. I worked around this by pulling the information out of the event object itself, but it would be nice if we could somehow recover this.

  • A very minor point: CompilerState returns a list of modules which have been parsed. If it could also provide the FilePaths these were parsed from, then I could register those files as dependencies in the Template Haskell calls.

But these are all relatively minor points. The fact that, without too much difficulty, I could convert some live Javascript code into Haskell code is very exciting. Maybe 2013 will be the year of Haskell on the browser :).


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