A couple nights ago I walked down to LispNYC in the East Village to hear Rich Hickey talk about Clojure, his new Lisp-like language. To be honest, I wasn’t expecting much. Another Lisp? Ho hum. I’m sure it’s very clever and cool and all, but not something I can actually use.

Instead, I was blown away by Rich’s presentation. Clojure might just be the Lisp I’ve been waiting for. Here’s why:

Clojure targets the JVM. That doesn’t just mean it’s written in Java. It compiles directly to Java bytecode, so it can take advantage of all the optimization work that’s been done on just-in-time bytecode compilers. So it’s both fast and portable. Clojure is functional, i.e. “side-effect free.” The built-in data structures are immutable. At the same time, you can have side effects (via external Java calls) without needing monads. Clojure has modern data structures. Vectors and hash maps are built in, and both can be used as “functions” that take a key value as their argument (I immediately thought yes, that’s the way it should be). Clojure fits into the Java ecosystem. All squintillion Java library methods can be called directly from Clojure. (Hickey demonstrated a small Swing app with fewer lines than the Java version.) Clojure data structures implement the Java collection interfaces. Clojure has built-in concurrency support. I barely understand this, but Hickey has done his homework and built a sophisticated concurrency and transaction system right into the language. Clojure abstracts Lisp sequences. Standard functions like “map” and “filter” work on any sequence, not just lists. In fact, they work on any object that implements the Java “Iterable” interface. Clojure has metadata. Objects can have metadata attached to them, which can be used by the program without affecting the value of the data (e.g. two objects with identical contents but different metadata are still considered “equal”). Clojure is still Lisp! It has first-class functions, Common Lisp-style macros (with automatic gensyms), a reader, “eval”, and a REPL.

I haven’t been this excited about Lisp since Peter Seibel came to talk about Practical Common Lisp. Hickey’s full presentation should show up soon at LispNYC.org. There’s also a (beautiful) Clojure web site and a Google group.

Update Nov. 21: Audio and slides from the presentation are available at LispNYC.