code-as-data.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