Hands-on with Clojure day 4

:: Clojure, Hacker School


  • Spent time with Clojure Cookbook.
  • Started a cheat sheet for Racket ↔ Clojure.
  • Looked at split-with.
  • Started a port of wffi from Racket to Clojure.
  • Confusion: Aggregates and generics.

Clojure Cookbook

I discovered Clojure Cookbook, which describes itself as:

Clojure Cookbook doesn’t just teach you Clojure, it also shows you how to use the language and many of its common libraries. The most difficult part of mastering any language is knowing how to apply it, in an idiomatic way, to tasks that real software developers encounter every day. This is especially true of Clojure.

With code recipes that teach you how to use the language in a variety of domains, Clojure Cookbook goes beyond simply teaching Clojure syntax and semantics. It contains annotated example code with detailed analysis and explanation for hundreds of real programming tasks. You can read the book straight through to gain insights about Clojure, or use it as a reference to solve particular problems.

You can git clone the Clojure Cookbook project and work with it in Emacs. This is a nice experience.

You can read each section in an adoc-mode buffer, with cider-repl active. That way you can C-x C-e to see the result on the Emacs status bar (or if you prefer, paste or type directly in the the cider-repl buffer).

Plus you can bind a key like M-+ to an Elisp function that finds and advances to the next section.

I found most sections to be helpful. Not only do they show how to do X, they get into the rationale of when and why should use a certain Clojure approach vs. another. Very good stuff.

Racket ↔ Clojure Cheat Sheet

As I’ve learned Clojure equivalents for Racket functions, I’ve jotted them down in notes. The notes got long enough that I moved them into org-mode tables — resulting in this cheat sheet.

The first part covers, given Racket function X, what’s the exact or close equivalent Clojure function Y? The second part is for the other direction, from Clojure to Racket.

Again, these are my rough working notes. Don’t rely on it. I wouldn’t be shocked if some of it is subtly — or blatantly — wrong.

I found that, as is often the case with taking notes, the act of taking them makes it less-likely to need to refer to them.

Cheat sheets, split-with

Speaking of cheat sheets, on #clojure someone reminded me of the Clojure cheatsheet. This seems like something to keep open in a browser tab. Or, say, print and paste on the inside of my forehead.

I was on IRC asking if Clojure had some equivalent to Racket’s splitf-at, with an implementation faster than the conceptual one I was able to come up with:

(defn split
  "FIXME: This is the conceptual, inefficient implementation. Should
  re-implement like Racket's splitf-at."
  [pred coll]
  [(take-while pred coll)
   (drop-while pred coll)])

Someone pointed out split-with, which I could have found on the cheat sheet.

So that’s cool. However M-. on split-with shows an implementation that is… the conceptual one. I guess that could be a micro-project — write an implementation that doesn’t excessively traverse and create intermediate collections.

wffi for Clojure

A couple years ago, I was reading so much Amazon Web Services documentation that included what were essentially HTTP request and response templates. Such as the following, albeit much more complicated than this:

GET /users/{user}/?key={val}
Header: {value}

Eventually it dawned on me that it would awfully be nice if web service documentation like this could actually drive code generation. Both generate client wrappers to make requests to the service, and, generate API glue for the service’s server. That was the idea behind webapi-markdown. For example, you could take some service’s documentation, run it through a tool, and voila, an “SDK” in your favorite language.

So webapi-markdown is just a spec. The one implementation to-date is in Racket: wffi. (The name implying both “web FFI” and “stinky”.)

In searching for a somewhat more challenging project to do in Clojure, I figured this might fit the bill.

So I spent the last day working on it. It’s been slow going, but so far I’ve had a chance to learn:

  • How to use markdown-clj for parsing markdown. Although it returns an HTML string, which isn’t ideal, it’s easy enough to convert that using tagsoup into structured data. (I keep thinking of these as “Clojure’s equivalent of x-expressions”. Is this known as “hiccup”? Not sure.)

  • How to use Instaparse. I only got a couple hours into this, but so far it seems pretty spiffy. It was fun to see the acknowledgment in the README that Danny Yoo’s Ragg was the inspiration. It is definitely nice to use in the same way that Ragg is, and more.

And eventually this project will get into areas like:

  • Code-generation via Clojure macros.

  • Generating docs (via doc strings, I suppose).

  • Distributing a project on Clojars (if I take it that far).

So I think that’s a pretty good chunk of real-world stuff, which makes it worth pursuing this, even if there’s no value in releasing it per se.

The code is still pretty raw, and my grammar for Instaparse needs work. So I’m making commits, but not confident enough to push stuff to GitHub quite yet.

Aggregates and generics

One area I’m still confused by is Clojure’s variety of approaches to aggregate data types and polymorphism.

Coming from Racket, we have structs and generics. structs have compile-time defined fields. As a result:

  • They’re fast: A field is a direct offset.
  • They’re strict:
    • You can’t accidentally add or omit a field.
    • Structs can inherit fields from other structs, strictly.

Of course we also have dictionaries (hash tables and association lists) when we want runtime flexibility.

My current impression is that Clojurians would often use a map instead of a struct in many situations. I think?

Clojure does have things that seem to be in the same ballpark as structs. defrecord sounds like it is about structs. Is it sugar for maps? deftype is, erm… I don’t know. Clojure Cookbook has a quote from Chas Emerick:

Is your class modeling a domain value—thus benefiting from hash map-like functionality and semantics? Use defrecord.

Do you need to define mutable fields? Use deftype.

Hmm. That’s not what I would have guessed from the name, “type”. Plus, I thought mutability was strongly discouraged in Clojure.

The docs for defrecord and deftype seem a bit terse and opaque to me: The words make sense but aren’t crisply connecting to concepts I already know.

So, clearly I need to learn more in this space.

Requires are less often, but still, tricky

Although it’s much better, I still seem to get confused by requires. Good example is clj-tagsoup. I kept trying to :require pl.danieljanus.tagsoup.core. But it’s just pl.danieljanus.tagsoup. The classpath error message isn’t very helpful for me. If I go in ~/.m2 and look, I don’t see source files. I need to go up on GitHub and view the source. That’s how I eventually noticed it’s src/pl/danieljanus/tagsoup.clj. Not e.g. src/pl/danieljanus/tagsoup/core.clj.

To be fair, I suppose this is really about one library’s choice. And I’m new to Clojure. But I don’t seem to struggle with this in Racket. Especially when an installed package has Scribble-generated docs, the docs automatically show a (require foo) that, well, just works the first time.

Reflection and next steps

Notwithstanding my whinge in the previous section, I’m hitting tooling and cider speed bumps much less frequently. I felt like yesterday was the first time I could actually focus on the code, without distraction, for solid hours at a time. Although I still don’t feel like I’ve figured out an optimal Clojure workflow, it is easier for me to imagine such a thing being possible someday.

Next up, I plan to keep working on the wffi port, and see how far I can take that this remainder of this week.