Issue: GET-SETF-METHOD-ENVIRONMENT (Version 5)

[Masinter.pa@Xerox.COM]

This issue passed conditionally. In this version, I  changed  the last
paragraph of the proposal,  and attempted to add a test case. I hope
that I did not spoil the intent. 
!
Issue:          GET-SETF-METHOD-ENVIRONMENT
References:     GET-SETF-METHOD (CLtL p 187)
Category:       Change
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
                
Problem Description:

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 ...) ...)

then  

 (macrolet ((test (x) `(car ,x)))
         (special-incf (test z)))

would not "see" the test definition.

Proposal (GET-SETF-METHOD-ENVIRONMENT:ADD-ARG):

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.

Test Case:

;;; 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)))
               ,setter)))))

(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 time

(flet ((frob (x) (cdr x))
     (xpop (frob z)))


Rationale:

This was an omission in the original definition of CLtL.

Current Practice:

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.

Adoption Cost:

Some implementations will have to add this feature, although it is not a
major change.

Benefits:

This change improves portability and the ability to use MACROLET, FLET
and LABELS in portable code which might also have SETF forms.

Conversion Cost:

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.

Aesthetics:

SETF methods cannot work correctly within lexically defined function
symbols without this change. This change makes the language more
consistent and correct. 

Discussion:

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 simplest.