Reading Clojure

Preamble

Back in early December I held a Clojure workshop for the Bucharest Functional Programming meet up. Having to explain the language to developers who were completely new at it was an interesting experience. I decided to start from the ground up: how to even read the blasted thing.

Branching workflow: git-flow and github-flow

Choosing a branching model for macchiato

Lately, I’ve been working on Macchiato to bring web application development libraries for ClojureScript on Node.js. Get a few people of different backgrounds involved on a project, and pretty soon a discussion about methodology will emerge.

Since both Dmitri Sotnikov and myself are working on some libraries at the same time, we had to decide which approach to use.

There are two major alternatives: Git-flow and Github-flow (with Gitlab-flow being a slightly more elaborate version).

Let’s review them.

HTML parsing in Clojure

Once again I’m doing a personal project that requires me to do some HTML parsing. If you have had to look at it, the landscape of Clojure HTML parsing libraries seems to be littered with dead projects.

Let’s look at the options.

Deploying a Clojure application from Gitlab CI to Heroku

Now that we have our Clojure application tested every time we push it to Gitlab, let’s configure another stage to deploy it to Heroku.

This assumes that:

  • Anything you merge to master is ready to deploy,
  • You have your Heroku API key handy, and
  • You already have a Heroku app configured, with all the necessary environment variables.

There’s two ways we can do this.

Using Gitlab CI with a Clojure project

Gitlab allows you not only to have free private repositories, but also to test them using free runners. These can run automatically, on push, for any branch or tag.

I keep a few private repositories with them, for personal projects and small experiments. I decided to give Gitlab CI a shot for a PostgreSQL-backed Clojure project.

There’s a basic example on the Gitlab CI repository. It gets and installs lein, which isn’t necessary. Instead, we’ll build use the clojure:lein Docker image.

Clojure, Cursive and Emacs

J. Pablo Fernández has recently posted a piece with the incendiary title of “Emacs is Hurting Clojure“.

I disagree with the idea behind the title, but then again, he seems to do so himself. He promptly clarifies:

Tip: Uncaught Invariant Violation

Uncaught Invariant Violation: onlyChild must be passed a children with exactly one child.

Are you using Reagent and Bootstrap with ClojureScript, and getting this error?

In my case, the cause was that I had a when inside an OverlayTrigger. Seems like a no-no. Further testing showed you can’t have an if either. Looks like OverlayTrigger can only wrap data, not something that might change.

In my case the solution was simple enough, once I figured out the cause: I just extracted when to wrap the OverlayTrigger, instead of the other way around.

Stop abstracting early

When I started working with Clojure, I wanted everything to be Clojure.

HTML? Vectors through Hiccup.
Configuration? Clojure maps.
SQL queries? Clojure lists.

After all, I might one day need to take advantage of Clojure’s functional abstractions to… um… well… I might need it, right?

Learning Clojure Fast

There have been several questions floating lately to the effect of “How do I get started fast?” or “What’s the bare minimum I need to know to get hired?”.

In case you’re wondering how to do that… you’re not going to like this answer.

ClojureScript tip: mind the Clojure version

Updated the ClojureScript version, and now you’re getting build errors? Check the Clojure version.

If you do an upgrade through lein ancient update, it will not upgrade the Clojure version. But if you have a ClojureScript dependency, it will be upgraded, and you may run into a case like this when building:

No such var: string/index-of, compiling:(cljs/source_map.clj:260:54)

I got this error when going from ClojureScript 1.8.34 to 1.9.93. I wasn’t using index-of directly, which is a Clojure 1.8 function… but ClojureScript was.