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Issue SETF-MACRO-EXPANSION Writeup

Issue:         SETF-MACRO-EXPANSION

References: DEFSETF, DEFINE-SETF-METHOD, SETF

CLtL pages 97, 102, 105

ANSI CL spec 6-162 (Sep 89 version)

Related issues: FUNCTION-NAME:LARGE

Category: CLARIFICATION

Edit history: 29-Apr-90, Version 1 by Moon

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 (SETF-MACRO-EXPANSION: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.

Esthetics:

Consistency.

Discussion:

None.


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