Cleanup Issue SETF-MACRO-EXPANSION

Category
CLARIFICATION
References
DEFSETF, DEFINE-SETF-METHOD, SETF CLtL pages 97, 102, 105 ANSI CL spec 6-162 (Sep 89 version)
Related issues
FUNCTION-NAME:LARGE

Problem Description

CLtL is inconsistent because pages 102 and 105 say DEFSETF and DEFINE-SETF-METHOD can be used with a function or a macro, while page 97 says that SETF expands macros before it looks for setf-methods.

This is Symbolics issue #9.

Proposal (LAST)

Clarify that SETF (actually GET-SETF-METHOD-MULTIPLE-VALUE) only expands macros after exhausting all other possibilities other than expanding into a call to a function named (SETF reader). Clarify that it expands macros in the style of MACROEXPAND-1 rather than MACROEXPAND (this was point 11 of FUNCTION-NAME:LARGE and is repeated here for clarity only).

Examples

(defvar *foo-trace* nil)

(defmacro foo (x y) `(aref ,x ,y))

(defun (setf foo) (z x y) ;wrong (push `(,x ,y ,z) *foo-trace*) (setf (aref x y) z))

(defsetf foo (x y) (z) ;right `(progn (push `(,,x ,,y ,,z) *foo-trace*) (setf (aref ,x ,y) ,z)))

(macroexpand-1 '(setf (foo array i) (val)))

The last form should include a push onto *foo-trace*, rather than first macroexpanding (foo array i) into (aref array i) and then setf'ing that.

The defun form has no effect because SETF expands macros before expanding into a call to such a function, so this function will not be used.

Rationale

In the absence of this clarification, the language specification is not self-consistent. The example given above will not work in implementations that chose to believe CLtL page 97 rather than page 102.

The rationale for FUNCTION-NAME:LARGE point 11 suggests that we thought we had exchanged the two bullets on page 97, so that SETF looks for setf methods before it expands macros. However there doesn't seem to be any cleanup issue that explicitly addresses this.

Current Practice

Symbolics Genera looks for setf methods before expanding macros, I didn't check any other implementations.

Cost to Implementors

Trivial.

Cost to Users

None.

Cost of Non-Adoption

The Common Lisp language will be a joke.

Performance Impact

None.

Benefits

Consistency.

Aesthetics

Consistency.

Discussion

None.

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