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From: gyro@kestrel.edu (Scott Layson Burson)
Newsgroups: comp.lang.lisp,comp.lang.scheme,comp.sources.wanted
Subject: SETF of LET
Message-ID: <1991May22.072355.22077@kestrel.edu>
Date: 22 May 91 07:23:55 GMT
References: <1991May20.201244.21948@uicbert.eecs.uic.edu>
Organization: Kestrel Institute, Palo Alto, CA
Lines: 43

I have just come to the conclusion, after trying for several hours,
that it is not possible to write a SETF method for LET using
DEFINE-SETF-METHOD.  In case it's not obvious what that means, here's
one not-quite-right way to do it:

  (define-setf-method let (clauses &rest body)
    (let ((storevar (gensym)))
      (values '() '() (list storevar)
	      `(let ,clauses
		 ,@(butlast body)
		 (setf ,(car (last body)) ,storevar)
	      (car (last body))))))

So, roughly speaking, (setf (let ((x ...)) (car x)) 'foo) turns into 
(let ((x ...)) (setf (car x) 'foo)).

The problem with the definition above is that it potentially evaluates
the subforms of the last form in the body more than once; so

  (incf (let ((x ...)) (car (pop x))) 3)

turns into (effectively)

  (let ((x ...)) (setf (car (pop x)) (+ 3 (car (pop x)))))

I can fix this problem, but then I get a version that doesn't
correctly handle DECLARE forms immediately inside the LET being
SETFed.

The point is not that it can't be done -- as far as I know, it is not
hard to specify what the expansion of SETF of LET should be -- but
that it can't be done with the DEFINE-SETF-METHOD interface.

Why would anyone want this to work?  It's not unusual for macros to
expand into LET forms.  It's true, I could write SETF methods for each
such macro individually, but I don't see why I should have to.

So I'm surprised that CLtL2 doesn't specify (at least not that I've
been able to find) that SETF of LET should work.  Is this perhaps
simply an oversight, that should be brought to the attention of X3J13?

-- Scott
Gyro@Reasoning.COM