Question

Does someone know why the following produces the expected result – (2 4 6)

(defmacro mult2 (lst)
  (define (itter x)
    (list '* 2 x))
  `(list ,@(map itter lst))) 

(mult2 (1 2 3))

while I expected that this one would (with the list identifier)

(defmacro mult2 (lst)
  (define (itter x)
    (list '* 2 x))
  `(list ,@(map itter lst)))

(mult2 '(1 2 3))

Answer 1

That’s because the '(1 2 3) is expanded by the reader into (quote (1 2 3)). Since you only destructure one list in your macro, it won’t work as expected.

Some general advice: if you’re working in Racket you probably want to avoid using defmacro. That is definitely not the idiomatic way to write macros. Take a look at syntax-rules and, if you want to define more complicated macros, syntax-parse. Eli also wrote an article explaining syntax-case for people used to defmacro.