Musings about FP and CS

A log of my journey through FP and CS

Building a small homepage

by Clement Delafargue on May 4, 2017

Tagged as: haskell, clever-cloud.

After a giving a talk, I’m regularly asked for a link pointing to the slide deck. I used to publish an article on my blog, but that took too much time. Instead, I’ve created a small web page which lists my talks, with videos and slides. In the next post, I’ll show how I’m using the physical web to broadcast its URL while I’m in a conference.

You can have a look at the page https://clementd.cleverapps.io and at its source code on github.

Choosing a web stack

Since it’s a side project, I’ve decided to experiment with new tools. I’ve played a bit with rust and rocket.rs, but it’s not sufficiently mature for me (it requires nightly, and templating is only available through mustache or mustache-like languages). Now that Clever Cloud has native haskell support, I decided to use it. In the past, I’ve played with both snap and yesod, but for a simple application like that, I chose to go with scotty. For templating, I’ve used blaze-html.

Routing

Scotty provides a thin layer over WAI, mainly routing, as well as a context wrapper (which I don’t use).

Templating

Over the years using Play Framework, I’ve come to love its type-safe templating system, Twirl. Instead of viewing templates as chunks of HTML with placeholders, it lets you describe templates as functions taking data as input and producing HTML as output. It sidesteps all the common issues of template inheritance, helpers, etc as it allows you to abstract your templates with simple functions. Of course, since you choose the types of input data, your templates and their use is checked at compile time.

My biggest gripe with Twirl is that it’s an external DSL resembling HTML, but with specific exceptions, that are hard to understand without a good understanding of the underlying parser. In practice, when you stray from common uses, you just end up adding or removing spaces and blank lines until it compiles.

As always, Haskell-land already has the solution I’ve been dreaming for without knowing it exists. In the case of templating, there are two embedded DSLs, blaze-html and lucid. I’ve used blaze-html, but lucid seems to be even better. In both cases, basic functions allow you to create html blocks. HTML elements are monadic, so you can compose them with do blocks. It’s not the purest use of monads I know, but it allows to declutter the resulting code.

Since I wanted a mobile-friendly page, I’ve used Material Design Lite to get a material design look and feel. The HTML you have to write however, is not really lite. That’s where haskell’s abstraction capabilities come in handy.

Stats gathering

Just for fun, I’ve decided to use EKG, haskells metrics gathering system. My first try used EKG’s HTTP interface for metrics exposition, but this pulled snap-core, and it felt a bit… too much. I’ve decided to push metrics trough the init protocol instead.

Since scotty is based on WAI, I was able to use wai-middleware-metrics, to gather web-related metrics (hit count, responses count, latency distribution, usw…).

The cool thing is that these metrics are automatically collected and gathered by Clever Cloud :-)

Development environment

Both snap and yesod provide auto reload dev environment, but raw warp doesn’t. stack run --file-watch does auto compilation, but it kills and reloads the server, which doesn’t really provide a save-refresh cycle. Thankfully, Guillaume Bort (from playframework’s fame), whose taste for simplicity matches mine, worked on dev loop. Directly inspired by play run, but not tied to any framework, it wraps the compilation phase and exposes the app behind a small reverse proxy. If compilation fails, it displays a nice error message in the browser. Another nice thing is how it handles environment variables: it starts the application with the right environment, whithout needing you to source it yourself. This way, you don’t mess up your shell.

Deploying on Clever Cloud

That’s the easiest part. Once you’ve made sure the generated application listens on $PORT, everything works out of the box: dependencies fetching (and caching), compilation, deployment.

clever create -t haskell my-homepage
clever deploy

Wrap up

I’ve played a lot with web stacks these last years, and now I know what I like: a small set of independent libraries working reasonably well together, providing

  • routing
  • templating if needed
  • serde (without reflection)

Scotty, aeson and blaze-html (or lucid) provide exactly what I need, on top of the rock-solid warp server. Thanks to WAI, there are lots of middlewares I can use to save time (metrics, CORS, usw).

Next time we’ll see how to broadcast a URL with the physical web API.

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