IGNORABLE

[Kent M Pitman <KMP@SCRC-STONY-BROOK.ARPA>]

    Date: Fri,  7 Mar 86 13:05:09 EST
    From: Jonathan A Rees <JAR@MC.LCS.MIT.EDU>
    
    Common Lisp (and in fact, every language that has variables) needs a
    (DECLARE (IGNORABLE ...)) in addition to (DECLARE (IGNORE ...)).  The
    meaning of (DECLARE (IGNORABLE X)) is: the variable X might or might not
    be used, and I don't want the compiler to generate a warning in either
    case.
    
    There is no way to simulate this feature in Common Lisp.  ...

While I agree that (DECLARE (IGNORABLE ...)) would be nice.

I dispute your contention that you can't simulate this feature. I always just write:

 (DEFMACRO WITH-FOO (&BODY BODY)
   `(LET ((FOO (COMPUTE-FOO)))
      FOO ;ignorable
      ,@(OR BODY '(NIL))))

I've never seen a reasonable Lisp compiler that didn't handle this correctly.
I would even argue that it is indirectly implied by the language specification
that this must work, and that it's a bug if a compiler warns either about
no FOO in body or a FOO in body.

-kmp

ps Please no discussion about what happens if BODY begins with a DECLARE form.
   The example above is obviously oversimplified in that respect, but adequate
   discussion of that issue has already taken place and isn't relevant here.