Difference between `(&rest xs)` and `xs` in defmacro formal argument list

Question

In Practical Common Lisp's Chapter 8, Macros: Defining Your Own, we define a macro with-gensyms as follows:

(defmacro with-gensyms ((&rest names) &body body)
  `(let ,(loop for n in names collect `(,n (gensym)))
    ,@body))

What is the purpose of the (&rest names)? If we replace it with just names, the effect seems to be the same. In both cases we pass in a list of symbols to be "gensym-ed".

Rainer Joswig · Accepted Answer · 2021-11-06T09:44:53.957

11

The difference:

(&rest names) inside the arglist ((&rest names) &body body) matches only lists. When somebody is using the macro with something else, the error comes from the macro expander:

debugger invoked on a SB-KERNEL::DEFMACRO-BOGUS-SUBLIST-ERROR:

error while parsing arguments to DEFMACRO WITH-GENSYMS:

bogus sublist A to satisfy lambda-list (&REST NAMES)

names matches anything. When somebody is using the macro with not a list, then the error comes from the LOOP macro, which needs a list:

debugger invoked on a TYPE-ERROR: The value A is not of type LIST.

Thus you get a better and earlier error.

edited Nov 06 '21 at 09:44

answered Dec 30 '15 at 10:42

Rainer Joswig

136,269
10
221
346

So it's idiomatic to use `((&rest args))` when we expect `args` to be a list? – Tianxiang Xiong Dec 30 '15 at 23:48
1

@TianxiangXiong: wouldn't you want to use the more specific construct? – Rainer Joswig Dec 31 '15 at 00:15

Difference between `(&rest xs)` and `xs` in defmacro formal argument list

1 Answers1