Reading Clojure, part 2

Welcome back

Have you gone over the fundamentals on Reading Clojure? Did you shake off that preconception that there’s some magic syntax to declaring and evaluating things beyond the list?

Great! Let’s now go over other things you might encounter when looking at a random source file. I’m first going to give you an overview of types and related things. After it, we’ll then we get into the good stuff like going over a project, and the weird stuff like ->, ->>, #, the quote and other squiggles.

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.

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:

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.

Complexity is the mind-killer

Chess moves

Last week I read an article from the MIT Technology Review that looked into what causes people to make mistakes. The approach was one I hadn’t seen before: it used data mining on a set of chess games. Their rationale was that:

  • There’s a huge database to feed your model from,
  • Chess is a deterministic domain where you can objectively evaluate good and bad moves,
  • There are clear criteria for the skill level of the participants, even when disparate, which allows you to balance.

As spoiled by the title, complexity was the main factor whenever a player made a mistake.

The bottom line is that the difficulty of the decision is the most important factor in determining whether a player makes a mistake.

So what’s the big deal?