A Lisp macro sees the actual source it encloses and this source does not need to adhere to any pre-defined syntax. For example: we may want a macro which outpu

A Lisp macro sees the actual source it encloses and this source does not need to adhere to any pre-defined syntax. For example: we may want a macro which outputs the source it encloses, before execution: CL-USER 12 > (defmacro my-loop (&body things) `(progn (pprint (list :doing-loop-with ',things)) (loop ,@things))) MY-LOOP CL-USER 13 > (my-loop for i from 1 upto 3 sum (my-loop for j upto 3 sum (* i j))) (:DOING-LOOP-WITH (FOR I FROM 1 UPTO 3 SUM (MY-LOOP FOR J UPTO 3 SUM (* I J)))) (:DOING-LOOP-WITH (FOR J UPTO 3 SUM (* I J))) (:DOING-LOOP-WITH (FOR J UPTO 3 SUM (* I J))) (:DOING-LOOP-WITH (FOR J UPTO 3 SUM (* I J))) 36 A normal function can't do that, since it does not get source as an argument, but just sees the arguments values. Since a macro sees source, it can not only print it, but also simplify, transform, mutate, etc. the source.