References: GET-SETF-METHOD (CLtL p 187)
Edit History: Version 1 9-Jan-87, Version 1 by Masinter
(no version) 7-Apr-87, merged with ENVIRONMENT-ARGUMENTS
Version 2 29-May-87, extracted again
Version 3 5-Jun-87, by Masinter
Version 4 11-Jun-87, for release
Version 5 13-Jul-87, by Masinter
If a macro that performs similar processing to SETF uses GET-SETF-METHOD,
and that macro occurs within a MACROLET, the expansion will not see the
MACROLET definition, e.g.,
(defmacro special-incf ... (get-setf-method ...) ...)
(macrolet ((test (x) `(car ,x)))
(special-incf (test z)))
would not "see" the test definition.
Add an optional environment argument to GET-SETF-METHOD and
GET-SETF-METHOD-MULTIPLE-VALUE. If the argument is not supplied, it
defaults to the null lexical environment. NIL can also be passed explicitly
to denote the null lexical environment.
Allow &ENVIRONMENT variable to appear in the lambda-list subform of a
DEFINE-SETF-METHOD form, as with a DEFMACRO.
Note that macros defined with DEFINE-MODIFY-MACRO correctly pass the
environment to GET-SETF-METHOD.
Clarify that, within the scope of a MACROLET, FLET and LABELS, global SETF
definitions of the name defined by the MACROLET, FLET or LABELS do not
apply. A SETF method applies only when the global function binding of the
name is lexically visible. All of the built in macros of Common Lisp
(SETF, INCF, DECF, POP, ROTATEF, etc.) which modify location specifications
obey this convention.
;;; This macro is like POP
(defmacro xpop (place &environment env)
(multiple-value-bind (dummies vals new setter getter)
(get-setf-method place env)
`(let* (,@(mapcar #'list dummies vals) (,(car new) ,getter))
(prog1 (car ,(car new))
(setq ,(car new) (cdr ,(car new)))
(defsetf frob (x) (value)
`(setf (car ,x) ,value))
;;; The following will modify (cdr z) and not (car z)
(macrolet ((frob (x) `(cdr ,x)))
(xpop (frob z)))
;;; The following is an error; an error may be signaled at macro expansion
(flet ((frob (x) (cdr x))
(xpop (frob z)))
This was an omission in the original definition of CLtL.
Many Common Lisp implementations already have this extension, although some
do not. One implementation has extended GET-SETF-METHOD to take an optional
argument which is incompatible with this use.
Cost to implementors:
Some implementations will have to add this feature, although it is not a
Cost to users:
This is generally an upward compatible change. In implementations which did
not already take into account the lexical environment for SETF'd forms
might start working differently if the internal implementation of SETF is
changed. The likelihood of this affecting a user's program is very small.
This change improves portability and the ability to use MACROLET, FLET and
LABELS in portable code which might also have SETF forms.
SETF methods cannot work correctly within lexically defined function
symbols without this change. This change makes the language more consistent
The cleanup committee generally supports this change.
A number of additional changes for rationally dealing with lexical
environments as first class objects, including a more general set of
accessors and constructors for lexical environments is required for many
language extensions (e.g., a portable version of the proposed Common Lisp
Object System) and should be addressed by a future proposal. For a while,
the cleanup committee attempted to deal with these issues together, but
decided to separate them out into their component parts. This issue is the