perl quantum weirdness explained

quantum_weirdness.el

;;;This gist refers to the problem referenced at http://www.perlmonks.org/?node_id=369247
;;;written in lispy format for clarity; the following is not necessarily valid lisp
;;;m is a variable currently equal to 20
;;;expression to be evaluated: (++m + m++)

;;;pretend pre-increment and post-increment exist in lisp - then, this is the expression:
(+ (++m) (m++))                  ; m = 20
;;; `preincrement' increments the variable and returns the variable
(+ m (m++))                      ; m = 21
;;; the following line is the magic - replace `postincrement' with `increment, then return original value'
(+ m (progn (setq m (1+ m)) 21)) ; m = 21
;;; setq does its job and there's only a return value left
(+ m (progn 21))                 ; m = 22
;;; return the 21
(+ m 21)                         ; m = 22
;;; time to evaluate +, so cash out the variable
(+ 22 21)                        ; m = 22
;;; the return value
43                               ; m = 22