In Common Lisp, how to define a macro expanding to nothing (rather than `nil`)?

Question

I want to define a macro which can comment a s-expression, for example:

I hope that

(list 1 2 (comment-macro (something))) -> (1 2)

But if I define the macro like this

(defmacro comment-macro (x))

the result of the above form is actually

(1 2 nil)

Answer 1

You cannot accomplish what you want with a regular macro because its primary value (or nil if no values are returned) will always be used.

However, there are two common options:

1. comment: #| ... |# - for general text

2. feature expressions: #+(or) ... or #-(and) ... - for (temporarily?) disabling code

You can also define your own read macros using set-macro-character and set-dispatch-macro-character.

Answer 2

In Common Lisp, there is no way to define a macro which expands to nothing. The primary value returned from the macro (i.e. the macro function) is always inserted in place of the macro call.