How does clojure’s syntax-quote work?

I don’t think there’s a syntax-quote equivalent of the quote function.

The Clojure reader is (currently) written in Java. The SyntaxQuoteReader class in src/jvm/clojure/lang/LispReader.java in the Clojure source is probably what you’ll want to read. It seems rather complex. You can see it building lists like (seq (concat ...)) there.

                ret = RT.list(SEQ, RT.cons(CONCAT, sqExpandList(seq)));

It’s common for the reader not to return straightforward Clojure code, but rather do the right thing in Java-land immediately. For example '[1 2 3] doesn’t yield the Clojure code (vector 1 2 3). Maybe it could work that way somehow, but it doesn’t. The reader just creates and returns the vector object itself.

Likewise, the SyntaxQuoteReader does some magic in Java immediately to resolve symbol namespaces and create gensyms itself and it returns some mangled and complicated-looking Clojure code that does the right thing, but isn’t necessarily easy for a human to read. Whether it’s like this because it has to be, or because it’s easier to do it this way in Java, or for performance or some other reason, I don’t know. Likewise I don’t know if quasiquote could exist as a plain macro/special form in Clojure and doesn’t, or if it couldn’t exist at all. I don’t see why it couldn’t though.

WrappingReader in the same file is the class that handles ' (plain old quote). You can see that it just wraps whatever you pass it in a list containing the symbol quote plus your argument. It’s much simpler. Note that this class also handles @, so that '@foo does return (deref foo).

This thread might shed some more light.

Edit

Here’s a proof-of-concept quasiquote macro. Note that this code is relying upon and abusing Clojure internals in a horrible way. Please don’t use this for anything.

user> (defmacro quasiquote [x]
        (let [m (.getDeclaredMethod clojure.lang.LispReader$SyntaxQuoteReader 
                                    "syntaxQuote" 
                                    (into-array [Object]))]
          (.setAccessible m true)
          (.invoke m nil (into-array [x]))))
#'user/quasiquote
user> (let [x 123] `(x 'x ~x))
(user/x (quote user/x) 123)
user> (let [x 123] (quasiquote (x 'x ~x)))
(user/x (quote user/x) 123)