Clojure: What is the meaning of ` and ~@?

Question

I am working through the problems at 4Clojure.

I have a working solution for the Tic-Tac-Toe exercise, but I can't understand Darren's solution:

(fn [b]
  (some (fn [p] (first (keep #(if (apply = p %) p)
                         `(~@b                   ; <- What is that ` and ~@?
                           ~@(apply map list b)  ; 
                           ~(map get b [0 1 2])
                           ~(map get b [2 1 0])))))
     [:x :o]))
 ;b is a two-dimensional vector

What is the meaning of ` and ~@?

Answer 1

` is the syntax-quote, which is used to write code as data without evaluating it. Note that it is clever enough to resolve symbols that represent functions to the correct namespace.

Examples:

`(+ 1 2)
=> (clojure.core/+ 1 2)    ; a list containing the + function and two arguments

(eval `(+ 1 2))
=> 3                       ; the result of evaluating the list

~@ is the unquote-splicing operator, which will enable you to expand a list of elements within some quoted data/code.

Examples:

(def args [3 4 5 6])

`(+ 1 2 ~@args 7 8)
=> (clojure.core/+ 1 2 3 4 5 6 7 8)

`(+ ~@(range 10))
=> (clojure.core/+ 0 1 2 3 4 5 6 7 8 9)

More detail on both of these and related operations can be found as part of the documentation for the Clojure reader.

Answer 2

See the "Syntax-quote" bullet-pointed section and examples in the documentation on the reader.

Answer 3

Even though it is Common Lisp, and not Clojure, the chapter on macros in Practical Common Lisp has some good examples that translate well:

http://www.gigamonkeys.com/book/macros-defining-your-own.html