What is the difference between defn and defmacro?

Question

What is the difference between defn and defmacro? What is the difference between a function and a macro?

Answer 1

Without sounding snarky, one creates a function, while the other creates a macro. In Common Lisp (and I'll assume this applies to clojure as well), Macros are expanded before actual compilation of functions. So, lazy-cat:

(defmacro lazy-cat
  "Expands to code which yields a lazy sequence of the concatenation
  of the supplied colls. Each coll expr is not evaluated until it is
  needed.

  (lazy-cat xs ys zs) === (concat (lazy-seq xs) (lazy-seq ys) (lazy-seq zs))"
  {:added "1.0"}
  [& colls]
  `(concat ~@(map #(list `lazy-seq %) colls)))

Will actually be expanded to

`(concat ~@(map #(list `lazy-seq %) colls)))

of which lazy-seq will then be further expanded to

(list 'new 'clojure.lang.LazySeq (list* '^{:once true} fn* [] body)))

all before the actually processing of the data passed to them.

There's a very cute story that helps explain the difference (followed by some informative examples) at Practical Common Lisp: Chapter 8