Issue GET-SETF-METHOD-ENVIRONMENT Writeup

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.


Cost to implementors:


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

major change.


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.


Benefits:


This change improves portability and the ability to use MACROLET, FLET and

LABELS in portable code which might also have SETF forms.


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.