Clojure Data As Code Example

example.clj

 ;; The code starts as a string in a text file, like any programming language.
  "(defn f [x]
    (* x x))"
​
  ;; Then it's read by the reader.
  (with-in-str
    "(defn f [x]
       (* x x))" (read))
​
  ;; Read returns this data structure:
  '(defn f [x]
     (* x x))
​
  ;; Which is a list of 4 elements,
  ;; Two symbols: defn and f
  ;; A vector which contains a symbol: [x]
  ;; And a list with three symbols: (* x x)
​
  ;; The function eval then transforms this data structure to jvm bytecode
​
  (eval '(defn f [x]
           (* x x))) ;;=> #'user/f
​
  ;; A nice consequence of this is that it allows macros
  ;; Which can be thought of as functions that take a code data structure
  ;; and returns another data structure that is then eval'd
​
  (defmacro rev [form] (reverse form))
​
  (rev (2 3 *)) ;;=> (* 3 2) ;;=> 6