Asterius GHC WebAssembly backend reaches TodoMVC

20 December 2018 — by Cheng Shao

Getting your browser to print “Hello World!” is fun, and a milestone for compiler writers targeting the web. Soon expectations change. Can you call existing code? Can you write a moderately complex web app? How fast does it run? Previously, we showed how Haskell can call JavaScript and vice versa using a foreign function interface. Today we demonstrate that writing web apps in Haskell compiled to WebAssembly works well enough that TodoMVC, the more intricate “Hello World!” of web apps, works in your browser. Try it out!. (Since our emitted code uses the BigInt feature, currently you need a recent version of Chromium to run it.)

Trying this at home

The complete source code of TodoMVC is included in asterius source tree here. You can try it online, or run it locally. As usual, the simplest way to do that is using our pre-built Docker image:

$ docker run -it --rm -v $(pwd):/mirror terrorjack/asterius
root@16758a0232ba:~/asterius# ahc-link --input-hs asterius/test/todomvc/todomvc.hs --browser
[INFO] Loading boot library store from "/root/asterius/.stack-work/install/x86_64-linux/ghc-8.7/8.7.20181115/share/x86_64-linux-ghc-8.7.20181115/asterius-0.0.1/.boot/asterius_lib/asterius_store"
[INFO] Populating the store with builtin routines
[INFO] Compiling asterius/test/todomvc/todomvc.hs to Cmm
[INFO] Marshalling from Cmm to WebAssembly
[INFO] Marshalling "WebAPI" from Cmm to WebAssembly
[INFO] Marshalling "TodoView" from Cmm to WebAssembly
[INFO] Marshalling "Main" from Cmm to WebAssembly
[INFO] Marshalling "ElementBuilder" from Cmm to WebAssembly
[INFO] Attempting to link into a standalone WebAssembly module
[INFO] Converting linked IR to wasm-toolkit IR
[INFO] Writing WebAssembly binary to "asterius/test/todomvc/todomvc.wasm"
[INFO] Writing JavaScript to "asterius/test/todomvc/todomvc.js"
[INFO] Writing HTML to "asterius/test/todomvc/todomvc.html"

After running the above commands, copy the asterius/test/todomvc directory somewhere else, run npm install in it to fetch common artifacts shared by all TodoMVC implementations, then you can fire up a local HTTP server and browse index.html to evaluate it.

Next, we’ll highlight some parts of our TodoMVC implementation, explaining improvements of asterius itself along the way.

Improved Haskell/JavaScript data marshalling

In our previous post, we demonstrated how to write functions to convert between Haskell types (e.g. String) and their JavaScript counterparts. Since the marshaling of certain types is such a common task, we now include them as a part of our standard libraries, and even implemented some of them as runtime builtins for better performance.

Check Asterius.Types for conversion functions between JSArrayBuffer/JSString/JSArray/JSObject/JSFunction types and their Haskell counterparts. Asterius.ByteString includes functions for converting between JSArrayBuffer and strict ByteStrings.

Previously, JSRef was still a magic type that isn’t defined anywhere. Now we renamed it to JSVal and made it less magical: it’s defined in Asterius.Types, and whenever you need to use it in a foreign import javascript declaration, you need to import that module. Also, it’s now possible to define and use newtypes for JSVal and other types in Asterius.Types; automatic wrapping/unwrapping in foreign declarations doesn’t work yet, but we’re actively working on that front.

DOM tree nodes as a datatype

An essential task in writing a TodoMVC implementation is building a part of the DOM tree that corresponds to the current app state. In vanilla JavaScript, we have stateful interfaces for this purpose. Since we are using Haskell, it would be nicer to add a layer of abstraction here: use a plain old datatype to model the DOM tree and encapsulate the logic of converting a “pure” tree to a “real” tree into one single function.

The ElementBuilder module contains our modeling:

data Element
  = Element { className :: String
            , attributes :: [(String, String)]
            , children :: [Element]
            , hidden :: Bool
            , eventHandlers :: [(String, JSObject -> IO ())] }
  | TextNode String

emptyElement :: Element
buildElement :: Element -> IO JSVal

The Element type allows us to create a DOM tree node, specify its class name and attributes, attach child nodes, and insert Haskell callbacks as event handlers. After using buildElement to convert it to a real JavaScript node, we can use interfaces like replaceChild/replaceWith to attach that node to the webpage.

Modeling and persisting TodoMVC app state

Another task is modeling and persisting app state. When the browser tab is reopened, we don’t want to forget our previous todo entries! Similar to how we model DOM tree nodes with a datatype, we’d also like to model the whole app state as a datatype:

data Todo = Todo
  { key, text :: String
  , editing, completed :: Bool
  } deriving (Generic)

instance Binary Todo

newtype TodoModel = TodoModel
  { todos :: [Todo]
  } deriving (Generic)

instance Binary TodoModel

loadModel :: String -> IO (Maybe TodoModel)
saveModel :: String -> TodoModel -> IO ()
modifyTodo :: TodoModel -> String -> (Todo -> Todo) -> TodoModel

Each todo entry is indexed by a randomly generated key, so later we can modify the content of a single Todo using modifyTodo. The todo model is simply a list of Todos, and it can be serialized using the binary package. Of course, we really want to use FromJSON/ToJSON here, and we can assure you that aeson support is definitely planned!

The loadModel/saveModel functions load/save a TodoModel value in localStorage. When our app starts, it tries to load the previous model first, and upon failure, falls back to an empty todo list as the initial state. We perform a saveModel whenever the current app state is changed.

Applying app state to the real world

Now, we’re equipped with datatypes to model our app state and the DOM tree, the remaining task is: perform the plumbing between the functional world and the real world.

Our method of applying pure state is quick and dirty: we implement a global store of our app state with IORef TodoModel, and a render callback which is invoked whenever the app state changes. render queries the current TodoModel, builds the DOM tree of the new todo list and replaces the old one, and also saves the TodoModel to localStorage.

We also need to create event handlers to process certain events: e.g. "click" events on buttons or "keypress" events on text input bars. To add handlers to DOM tree nodes created from our Element datatype, we just need to insert the Haskell callbacks into the eventHandlers field. As for the nodes that already exist as a part of the TodoMVC project skeleton, we implement a TodoView module containing bindings to the widgets we’re interested in, then we apply addEventListener on them in our main module. See the complete code of todomvc.hs for details.

Putting it all together, we arrive at a complete implementation of TodoMVC in Haskell: first, we perform a pure modeling of app state and the DOM tree, then we seek a way of receiving input from the real world and applying these pure models to it.

Of course, this implementation of TodoMVC is not yet satisfactory: we didn’t properly implement a virtual DOM with a decent diff algorithm to minimize actual mutation on the real DOM tree; we didn’t use a fancy FRP framework, and instead threaded some global state along the app using an IORef. However, it does show that it’s already possible to apply asterius to frontend development, enjoying both the performance of WebAssembly and the nice developing experience of Haskell.

Next steps

It’s been nearly a year since the asterius project started. Despite various difficulties, asterius grew from nothing to printing Cmm, to compiling fib, to handle JavaScript interop, and eventually, to a functioning TodoMVC. We’re on the edge of graduating from just research prototype. That means it’s now time to start communicating updates more regularly, with reports updated weekly and roadmaps reviewed quarterly, both included in the docs and available to the public.

And here is a list of improvements to wait for in the next quarter :)

  • Proper GC and exception handling. For garbage collection, a reasonable starting point would be a single-generation copying GC, from which we can gradually improve. Keep in mind that eventually WebAssembly runtimes will be exposing their own GC’s. For exception handling, we’ll enable users to handle Haskell and JavaScript exceptions in a uniform manner: a Haskell exception can be caught in JavaScript, and vice versa.
  • More comprehensive regression tests on CI, so we know with greater confidence what primops/runtime interfaces/standard library functions are working and what aren’t. We’ll provide that information as a status page so people can set reasonable expectations on the status quo of this project, and see how it improves over time.
  • Support for even more packages, up to aeson.

About the author

Cheng Shao

Cheng is a Software Engineer who specializes in the implementation of functional programming languages. He is the project lead and main developer of Tweag's Haskell-to-WebAssembly compiler project codenamed Asterius. He also maintains other Haskell projects and makes contributions to GHC(Glasgow Haskell Compiler). Outside of work, Cheng spends his time exploring Paris and watching anime.

If you enjoyed this article, you might be interested in joining the Tweag team.

This article is licensed under a Creative Commons Attribution 4.0 International license.


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