~/dnrvs Published: 2020-09-28 00:00:00 UTC

Learning Crystal

This week I came across Crystal and decided I liked the look of it enough to give it a go. I took a look around the ecosystem and spotted Kemal, a simple looking webserver with support for websockets. Websockets are something I had played around with in Rust and I wanted to see if I could accomplish something with them easier than I found it with Rust (though this was a while back and the ecosystem may have improved). The results of this exploration can be seen in this simple chat server: danreeves/crystal-lang-web-test.


Crystal is a compiled and statically type checked langauge with syntax inspired by Ruby. I’ve never used ruby but apparently people think it has nice syntax. So far I tend to think it’s nice in most ways but has some idiosyncracies I’m not used to, even as someone comfortable with lua and python.

The first problems I encountered was no built in “watch” command like I’m used to in Rust and when using webpack for frontend work. I looked at the solutions I could find and didn’t spot anything that was to my liking, mostly binaries that needed to be installed system wide and with custom yaml configuration files. I’m a fan of the way cargo watch builds right into the cargo CLI and provides a holistic approach. As a solution, and a decent excuse to learn how to write reusable modules in the crystal ecosystem, I wrote watch.


Watch is a simple Crystal script which lets you define multiple file globs to watch over with commands assigned to be run on changes.

require "watch"

Watch.watch "./**/*.cr", "crystal src/server.cr", opts: [:verbose, :log_changes, :on_start]


Since the default operation of the crystal compile CLI is to run the file you can now run crystal watch.cr and it’ll watch the /src directory, managing the starting and ending of processes and piping stdout and stderr for you.

It’s quite simple and probably highly inefficient at scale but it works for my simple purposes.

The concurrency model of Crystal, fibers, enable multiple processes to be run with highly readable code.

The rewrite

In time honoured tradition, the next thing for me to do was rewrite this site. Using Kemal again, but this time with Temel for the templating. Temel creates a simple DSL for generating markup, so my view route handlers look something like:

get "/:page" do |env|
  page = env.params.url["page"]
  markdown = File.read("./pages/#{page}.md")
  html = Markd.to_html(markdown, Markd::Options.new(smart: true))
      "#{page} | dan reeves"

I like the simplicity of Crystal, Kemal, and Temel over the combination of actix-web and maud I was using in the Rust version. The compile times are much faster. I was able to rewrite the a lot of what I needed without bringing in many dependencies because Crystal has a pretty broad standard library.

Handy resources: