Wednesday, January 7, 2015

One Major Difference Between Clojure And Common Lisp

In the summer of 2013, I attended an awesome workshop called WACM (Workshop on Algorithmic Computer Music) at the University of California at Santa Cruz. Quoting from the WACM site:

Students will learn the Lisp computer programming language and create their own composition and analysis software. The instruction team will be led by professor emeritus David Cope, noted composer, author, and programmer...

The program features intensive classes on the basic techniques of algorithmic composition and algorithmic music analysis, learning and using the computer programming language Lisp. Students will learn about Markov-based rules programs, genetic algorithms, and software modeled on the Experiments in Musical Intelligence program. Music analysis software and techniques will also be covered in depth. Many compositional approaches will be discussed in detail, including rules-based techniques, data-driven models, genetic algorithms, neural networks, fuzzy logic, mathematical modeling, and sonification. Software programs such as Max, Open Music, and others will also be presented.


It was as awesome as it sounds, with some caveats; for instance, it was a lot to learn inside of two weeks. I was one of a very small number of people there with actual programming experience; most of the attendees either had music degrees or were in the process of getting them. We worked in Common Lisp, but I came with a bunch of Clojure books (in digital form) and the goal of building stuff using Overtone.

I figured I could just convert Common Lisp code almost directly into Clojure, but it didn't work. Here's a gist I posted during the workshop:


This attempt failed for a couple different reasons, as you can see if you read the comments. First, this code assumes that (if (null list1)) in Common Lisp will be equivalent to (if (nil? list1)) in Clojure, but Clojure doesn't consider an empty list to have a nil value. Secondly, this code tries to handle lists in the classic Lisp way, with recursion, and that's not what you typically do in Clojure.

Clojure's reliance on the JVM makes recursion inconvenient. And Clojure uses list comprehensions, along with very sophisticated, terse destructuring assignments, to churn through lists much more gracefully than my Common Lisp code above. Those 7 lines of Common Lisp compress to 2 lines of Clojure:

(defn build [seq1 seq2]
  (for [elem1 seq1 elem2 seq2] [elem1 elem2]))

A friend of mine once said at a meetup that Clojure isn't really a Lisp; it's "a fucked-up functional language" with all kinds of weird quirks which uses Lisp syntax out of nostalgia more than anything else. To me, this isn't enough to earn Clojure that judgement, which was kinda harsh. I think I like Clojure more than he does. But, at the same time, if you're looking to translate stuff from other Lisps into Clojure, it's not going to be just copying and pasting. Beyond inconsequential, dialect-level differences like defn vs. defun, there are deeper differences which steepen the learning curve a little.