Hands-on with Clojure day 3

:: Clojure, Hacker School

Please see the usual disclaimers from my previous posts.

As I mentioned yesterday, my next toy project is to write wrappers libraries for the new Hacker News API. This seems like a good exercise because the REST API is very simple, and I have experience doing this sort of thing in Racket. In fact, I’ll do the same thing in both Racket and Clojure.

The result is clacker-news and racker-news. Trademark registration application is in-process.1

Although I knew this would be a simple project, it turned out to be simpler than I expected:

  • The Hacker News API is just a half-dozen items, so that helps.

  • Both Racket and Clojure have easy ways to parse JSON into the relevant idiomatic representations. (As you might imagine, Racket is more listy and Clojure is more vectory, but they are pretty similar. Because JSON.)

  • I didn’t try to do anything fancy with error-handling. If there’s an HTTP-related exception, it’s going to bubble up to the user of my library. I think this is probably fine.

  • I didn’t try to do anything fancy with retries. This is probably less-fine. Given response like say 429 Too Many Requests, it would nice if my library would automatically retry some number of times (with an exponential delay back-off). But it doesn’t.

However, maybe it was unduly easy — because I overlooked something important in Clojure (or about making REST API requests in general). If so, feel free to hit the comments.

Redefinitions and the top-level

While doing some copy-pasta of the Racket source to Clojure, I accidentally ended up with two defns of the same function, get-user. The thing is, Clojure did not give me an error message. This really surprised me. In Racket, this would be a redefinition error.

I guess this is related to my observation in yesterday’s post about the need to use declare for forward references. If I understand correctly, this means that Clojure’s evaluation model is closer to a simple load: It is essentially equivalent to typing stuff at a top-level REPL prompt. Things are evaluated one s-expression read at a time.

So for example it’s perfectly fine to redefine something at the top-level in the REPL.

And, if you read things one s-expression at a time — as opposed to an entire file and/or namespace — you can’t know about something that isn’t defined yet. You need a hint like declare: “Chill, I’m going to supply it later.”

What I’m used to from Racket is an actual module system. As I understand it, the first unit of evaluation is a module. And the #lang business is a shorthand for module forms. In other words:

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#lang racket
(define (id x)
  x)

is shorthand for:

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(module racket
  (define (id x)
    x))

And in fact you will see older Racket files that use the module form explicitly like that (as well as newer #lang files that use module forms within, i.e. for nested modules).

Maybe I’m misunderstanding, and the business about modules is orthogonal to the business about redefinitions and forward references. Maybe that’s really about read-ing s-expressions one at a time like in a top-level REPL. In any case, I don’t love this aspect of Clojure.2

Next steps

I need an idea for a gradually more-complicated project to try, next.

Ideally it would exercise something special/strong about Clojure, such as the persistent immutable data structures, concurrency primitives, or so on.

(As a counter-example, I could explore macros in Clojure, but my expectation is that’s an area that might be disappointing compared to Racket. Instead I’d like to find something where I’m more likely to say, “Dang, I wish Racket did this.”)

If you have suggestions, let me know in the comments, and thanks in advance.

  1. Kidding. 

  2. Among Racketeers, a famous quote is, “the top-level is hopeless”