Question

I tried to write a macro and execute it as follow. but it failed to execute.

(defmacro times_two (var) (* 2 var))
(times_two '(+ 1 2))

In my imagination, I think the expansion would be (* 2 (+ 1 2)). and after execution, the result would be 6. But failed.

I don’t know why. I read the Emacs lisp manual, but I still can’t understand them. I want to know what on earth the exact steps is while constructing expansion. What did the interpreter do?

Answer 1

When I evaluate these forms in Emacs, I get this error message when evaluating the second one:

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p (quote (+ 1 2)))
  *(2 (quote (+ 1 2)))
  (lambda (var) (* 2 var))((quote (+ 1 2)))
  (times_two (quote (+ 1 2)))
  eval((times_two (quote (+ 1 2))))
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp nil nil)

This is showing you how it expanded the macro, which should tell you what went wrong. (The final expansion is at the top.)

The quoted expression '(+ 1 2) gets passed to the times_two macro, but a quoted list is not a valid argument to the * function.

What you actually want here is:

(defmacro times_two (var) `(* 2 ,var))
(times_two (+ 1 2))

Keep in mind that, generally, the result of a macro will be new Lisp code, not a final value. Your goal in writing a macro is to construct the form that will give you the result you want. Thus, most of the time your macro will end up using the quasiquote (`) syntax.