Edge computing with servant on Cloudflare

9 October 2020 — by Cheng Shao

Just a few days ago, Cloudflare and Stack builders published a excellent blog post where they demonstrate how to deploy a Haskell program to Cloudflare Worker with Asterius. In this post we go one step further and show that you can deploy full-fledged servant applications just as well.

Suppose we’re about to write the backend of yet another web app. The traditional way of doing this is to implement a backend program that listens on a port, waits for incoming requests, and sends responses back. The backend program is deployed to a controlled server environment and runs persistently. This is the most flexible approach since there’s no limit on the tech stack: we can use any programming language and rely on arbitrary runtime dependencies. But of course, we’re also responsible for provisioning the deployment server, which is a hard and error-prone process. Even if we use cloud computing instead of managing a physical machine, we’re still in charge of jobs like monitoring the instance state, performing security updates, or upgrading to larger instances when necessary.

What if we could just forget about servers altogether? We just need to ensure the backend program conforms to a certain request handler function interface, then the cloud provider will run the handler function on a per-request basis without our intervention. This also changes the programming paradigm of backends; instead of long-running programs, they are now transient programs, started and terminated frequently, and the lifetime of a running instance shouldn’t be assumed to be much longer than the time it takes to process a single request. This is exactly what “serverless computing” promotes, and major cloud providers have already been providing this service for some time, like AWS Lambda, Azure Functions, Google Cloud Functions, etc.

Edge computing takes serverless computing even further, by running the deployed code in the CDN servers. The end user’s request reaches the CDN, computation occurs, and the result can be returned directly instead of having to forward the request to a further server and forward its response back to the end user. This means an improvement in overall latency since computation occurs close to the end user.

Compared to regular serverless computing, edge computing brings an additional challenge: tighter restrictions on the runtime environment. Ordinary serverless computing platforms offer a variety of choices for the backend programming language, and they sometimes even allow running native executables. Edge computing platforms usually restrict developers to a single scripting language for technical reasons: Cloudflare Workers supports JavaScript/WebAssembly, Lambda@Edge supports Node.js and fastly supports pure WebAssembly.

Asterius is a Haskell-to-WebAssembly compiler that supports a variety of existing Haskell packages. Since it emits WebAssembly & JavaScript code, it can be used to develop edge computing apps provided the target platform supports WebAssembly and JavaScript. Why would we want to run Haskell on the edge? Here are only a few of the possible answers:

  • to wrap popular Haskell apps like pandoc or ShellCheck into web services,
  • to adapt existing Haskell web backend codebases to benefit from edge computing, or
  • the same reason you choose Haskell for vanilla web backend projects, e.g. type-driven development.

As we explored potential use cases of Asterius, we’ve succeeded in deploying an example servant web app to Cloudflare Workers, and this post shows how we did it. You’re welcome to give it a try with your Worker program or other providers like Lambda@Edge.

What does a Worker function look like?

To use Cloudflare Workers, we need to write or generate a single JavaScript program that implements the Worker function. The Worker function is a fetch event handler. When there is an incoming HTTP request, a fetch event is fired, then the handler is called to perform a user-defined computation and produce the response:

addEventListener("fetch", event => {

async function handleRequest(request) {
  return new Response("Hello world")

To implement a Worker function in Haskell, the Haskell module needs to export the request handler function and the JavaScript entry script needs to set up the Asterius runtime, call the function, and get the result upon incoming requests.

A wai app as a Worker function

The idea of Request -> Response functions reminds us of something similar in the Haskell web development landscape. The wai framework provides this type:

import Network.Wai

type Application = Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived

The Application type defines a function type which consumes a Request and uses a Response afterward.

Haskell web backend developers should be pretty familiar with wai and the Application type. The type serves as a keystone in many Haskell web backends:

  • High-level web frameworks like servant and yesod encapsulate the business logic into a single Application which examines requests and reroutes to the appropriate handler based on the request URL.
  • One can apply Middlewares, which are simply Application -> Application functions to add specific functionality, like gzip encoding, logging, caching, etc.
  • An Application can be converted to a traditional web server program which polls requests on a port.

Given how central a role Application plays in Haskell web backend development, we should use it to model Worker functions. If it works, then in principle it should be possible to migrate higher-level backend codebases to Workers as well. The next section demonstrates adapting an Application as a Worker function, then we’ll move on to implement a minimal servant web app on top of it and deploy the example.

Bridging the wai world and the JavaScript world

Before we turn a wai-based app into a Worker function, we need to implement the conversion between the Request/Response data types in wai and the JavaScript Request/Response classes as described in the Worker documentation.

newtype JSRequest = JSRequest JSVal
newtype JSResponse = JSResponse JSVal

parseRequest :: JSRequest -> IO Wai.Request
makeResponse :: Wai.Response -> IO JSResponse
fromWaiApplication :: Wai.Application -> JSRequest -> IO JSResponse

Using parseRequest and makeResponse, we can implement fromWaiApplication which converts a Haskell Application to a JSRequest -> IO JSResponse event handler function to be exported and called in the JavaScript world.

The functions above are the infrastructure for running Haskell code as Workers, useful for any wai-based web framework. For the complete code, see here.

A servant app as a Worker function

Let’s move on to implement a minimal servant app as a proof-of-concept Worker function. The following module implements a small fraction of httpbin. The httpbin website provides a service for testing HTTP client libraries. One can make requests to its different endpoints, and it’ll return JSON-formatted client information. For instance, accessing /ip will return the end user’s IP address as a string.

The example below implements 3 endpoints: /ip, /country, and /user-agent, which return the visitor’s IP address, country, or user agent. The Worker runtime already encodes this information as HTTP request headers, so all we need to do is declare the required header field and return the value. There is no server or other third party services involved, therefore the example is self-contained and can be deployed to the Worker preview service without any credentials.

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeOperators #-}

module HttpBin (handleFetch) where

-- The complete source code of the Cloudflare.* modules is available at
import Cloudflare.Application
import Cloudflare.Request
import Cloudflare.Response
import Data.Text (Text)
import Servant

type HttpBin =
  "ip" :> Header "cf-connecting-ip" Text :> Get '[JSON] Text
    :<|> "country" :> Header "cf-ipcountry" Text :> Get '[JSON] Text
    :<|> "user-agent" :> Header "user-agent" Text :> Get '[JSON] Text

httpBinServer :: Server HttpBin
httpBinServer = f :<|> f :<|> f
    f (Just s) = pure s
    f _ = pure "Header not found."

httpBinApp :: Application
httpBinApp = serve (Proxy @HttpBin) httpBinServer

handleFetch :: JSRequest -> IO JSResponse
handleFetch = fromWaiApplication httpBinApp

foreign export javascript "handleFetch" handleFetch :: JSRequest -> IO JSResponse

The exported handleFetch function is the entry point of the Haskell world. For each incoming fetch event, the following JavaScript code will initialize an Asterius runtime instance, call handleFetch and respond with the result.

import * as rts from "./rts.mjs";
import req from "./HttpBin.req.mjs";

addEventListener("fetch", (event) => {
      .newAsteriusInstance(Object.assign(req, { module: wasm }))
      .then((i) => i.exports.handleFetch(event.request))

Compiling & deploying the Worker function

We’ll use the official Worker CLI wrangler to perform the deployment. Two config files are required. The wrangler.toml file sets up a webpack-based wrangler project:

name = "cloudflare-new"
type = "webpack"
webpack_config = "webpack.config.js"

account_id = ""
workers_dev = true
route = ""
zone_id = ""

The webpack.config.js file specifies the HttpBin.mjs entry script as the starting point of code bundling:

module.exports = {
  entry: "./HttpBin.mjs",

Now, we can finally compile the example and deploy it to the Worker preview service. The following commands work with the prebuilt container image of Asterius:

# --browser is required for Worker functions
# --yolo enables no-op GC, see next section for an explanation
# --gc-threshold=1 sets a smaller initial heap size
# --no-main is required since it's not a Main module

ahc-link                        \
  --browser                     \
  --yolo                        \
  --gc-threshold 1              \
  --input-hs HttpBin.hs         \
  --input-mjs HttpBin.mjs       \
  --export-function handleFetch \

# wrangler hard-codes the expected .wasm file path

mkdir -p worker
cp HttpBin.wasm worker/module.wasm

# Specify the /ip endpoint. Without --url the default endpoint is /

wrangler preview --url

The wrangler tool will bundle and deploy the JavaScript & WebAssembly code. The result will be available in the current console and an opened browser window:

Running preview without authentication.
Your Worker responded with: ""


The example in the last section is based on servant and wai. Other web frameworks should work as well, as long as we can wrap them into fetch request handler functions.

Though most frameworks should be straightforward to adjust, here are a couple of tips you might find useful when developing Worker functions:

  • Unlike ordinary web server programs which are expected to run for a long time, Worker scripts are started and stopped by need. Therefore, instance-level global state is discouraged since it’s fragile. Instead, the application state should be explicitly provided by an external service, e.g. a database or a message queue.

  • By not relying on instance-level global state, we’re also able to use a no-op GC which can increase the throughput of the Worker function. As demonstrated in the example, for each incoming fetch request, a new Asterius runtime is initialized to run the handler function. In the outer JavaScript world, the used Asterius runtimes are garbage collected shortly, so the heap space used by Haskell will not leak.

  • Here’s a tip on generating smaller WebAssembly code for deployment. In most cases, we need to handle multiple endpoints in a Worker function. Conventional web frameworks allow us to combine the handlers of different endpoints into a single one which picks the right handler at runtime based on the request URL. Another way to handle different endpoints is wrapping their handlers into different Worker functions, then it is more likely to generate smaller WebAssembly code due to dead code elimination of Asterius.

We believe the combination of Haskell, WebAssembly and edge computing has much potential. The initial Cloudflare Worker test was contributed by external contributor Ento on Github, many thanks! Feel free to reach us in case you find this use case interesting and want more in-depth support.

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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