lisp macros

1. Misc

Macros are special functions that transform an input lisp form with custom semantics to valid lisp forms.
This transformation is generally an expansion (I'm not aware of syntax contractions) and is addressed as a macro-expansion.
they don't evaluate their args completely but transform them into lisp forms that can be evaluated.
macro chaining is a thing : incf expands to setf which in turn uses the primitive setq (this is implementation dependent : alternatives exist)
point of difference from functions:
- arguments of a function are evaluated and bindings are made to local variables
- a macro transforms symbols/list of symbols into a valid form with selective evaluation as needed -> one can control what is substituted after evaluation in the final form : see backquote and comma.
- the output form of a macro is evaluated immediately.
- a functions output can be anything (need not be a form) and isn't evalutated
- note that special functions are the lowest level building blocks for control and structure, and are different than a function or a macro. eg: if, setq, let, block -> they can't be written above the implementation layer with a normal function/macro.
macro-destructuring can be used to dissasemble arguments into constituents in the unexpanded form itself.
- not possible for functions

(defmacro print-pair ((a b))
  `(format t "~&pair consituents : ~S ~S" ,a ,b))

(macroexpand '(print-pair (1 2)))

#+RESULTS:
| FORMAT | T | ~&pair consituents : ~S ~S | 1 | 2 |

all macros run at compile time :- they won't be an overhead at run time
do note the difference between a function and a macro
- they might result in the same final evaluation but the overhead for a macro is taken care of at compile time.
- a function producing an evaluable expression will be tending to this overhead at run time.