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From: skef@SPICE.CS.CMU.EDU (Skef Wholey)
Newsgroups: comp.lang.lisp
Subject: Re: Common Lisp Macro Expander Wanted
Message-ID: <1348@PT.CS.CMU.EDU>
Date: 7 Apr 88 22:11:05 GMT
References: <49@spar.SPAR.SLB.COM> <23539@ucbvax.BERKELEY.EDU>
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Organization: Carnegie-Mellon University, CS/RI
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Jim Larus writes:

    From: larus@paris.Berkeley.EDU (James Larus)
    Subject: Re: Common Lisp Macro Expander Wanted

    How about:

	    (defun macroexpand-all (f)
	      (cond ((atom f) f)
		    ((and (symbolp (car f)) (macro-function (car f)))
		     (macroexpand-all (macroexpand f)))
		    (t (mapcar #'macroexpand-all f))))

This is no good, because it doesn't know the syntax of special forms.
For example, this call causes an error:

  (macroexpand-all '(let ((with-open-file t)) blag))

because it tries to descend blindly into the LET binding list as if it
were a macro call, which it obvously ain't.

It isn't too hard to write a code walker that Does The Right Thing for
special forms in Common Lisp, but unfortunately some implementations
ignore the rules stated in CLtL and make this more difficult than it
ought to be.  Older versions of Symbolics CL were pretty nasty in this
respect, because they had macros expanding into implementation-dependent
special forms and didn't supply macroexpansion functions for all the
things specified as macros in CLtL.  I believe this should have been
fixed by now, but I can't easily check this.

I have this suspicion that dozens of good CL code-walkers have been
written independently, but none have made it into the public domain
because they were written in profit-oriented institutions.  Does anyone
out there have one to donate?

--Skef