guys.
Today I want to write sigma macro to calculate the sum from the flexible expression input.
The code below is I written this afternoon. But it does not work follow my purpose.
(defmacro sigma (exp ll)
`(+ ,@(loop for i in ll collect
(progn (setf (elt exp 1) i)
(print exp)
exp)))
)
>>(pprint (macroexpand-1 '(sigma (+ 1 2) (2 3 4))))
>>(+ 2 2)
(+ 3 2)
(+ 4 2)
(+ (+ 4 2) (+ 4 2) (+ 4 2))
I want it works (+ (+ 2 2) (+ 3 2) (+ 4 2)) but loop collect give me the weird answer.
Why does it work like this? Do I have some methods to fix this?
If you want a freshly consed list, then copy-list is a way:
(defmacro sigma (exp ll)
`(+ ,@(loop for i in ll and exp1 = (copy-list exp)
do (setf (second exp1) i)
collect exp1)))
Nested backquote expressions are also possible:
(defmacro sigma ((op arg0 &rest args) ll)
(declare (ignore arg0))
`(+ ,@(loop for i in ll collect `(,op ,i ,@args))))