CLHS: Issue GET-SETF-METHOD-ENVIRONMENT Writeup

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## 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.

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