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- To: common-lisp@su-ai.ARPA
- Subject: get-setf-method
- From: hpfclp!diamant@hplabs.ARPA
- Date: Thu, 6 Mar 86 17:54:10 pst
In the proposed clarifications for CLtL, for page 107, GLS states
that GET-SETF-METHOD should not do macroexpansion, and
that it should be called with an already expanded form.
The purpose of this change is to avoid having to make GET-SETF-METHOD
take an &environment parameter, since MACROEXPAND needs it.
He says "in most contexts the usage should be
(GET-SETF-METHOD (MACROEXPAND form env))." This seems
incorrect to me, as there may be a setf method defined on an intermediate
expansion of the macro, which should take precedence over the macroexpansion.
In order to correct this problem, each function which called GET-SETF-METHOD
would have to repeatedly call it with MACROEXPAND-1 until there were no
longer any macros to expand, or GET-SETF-METHOD found a setf method.
What would GET-SETF-METHOD return if it didn't find a setf method?
Here is an example of the problem:
(defsetf foo boat)
(defmacro foobar (x) `(foo ,x))
(defmacro foo (x) `(car ,x))
(setq x '(a . b))
(setf (foobar x) 3) ; this will expand to something equivalent to
; (setf (car x) 3) instead of calling boat as
; the update function.
In other words, it missed the boat!
In looking at the current SPICE sources, I see that GET-SETF-METHOD does
take an environment, and MACROEXPAND-1 is called repeatedly, rather than
MACROEXPAND. Due to the discrepency between the SPICE code and the
clarifications list, I assume that either no consensus has been reached,
or some decision has been made of which I am not aware.
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