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Issue SETF-OF-APPLY Writeup

Issue:               SETF-OF-APPLY

References: SETF



Edit history: V1, 10 May 90, Sandra Loosemore

V2, 29 May 90, Sandra Loosemore (forgot BIT & SBIT)

Problem description:

CLtL requires that (SETF (APPLY #'foo ...)) can be used only for

places "foo" whose SETF expansion is implemented in such a way that

the last subform of the accessing form appears as the last

argument of the storing call. The discussion and example imply that

the SETF method for AREF is required to implemented in this way. Are

there any other SETF places defined in the standard that are

required to be usable with APPLY?

This is Loosemore's issue #28.


Require that (SETF (APPLY #'<place> ...) ...) work for the standard

<place>s AREF, BIT, and SBIT, but document that the consequences of

using any of the other standard SETF <place>s with APPLY are


This proposal does not change the behavior of (SETF (APPLY ...))

with user-defined SETF methods; any SETF methods that are defined

by the user in such a way that the last subform of the accessing

form appears as the last argument of the storing call continue

to work with SETF of APPLY.


Of all the standard SETF places, AREF, BIT, and SBIT appear to be

the only functions that are defined with &REST arguments and where

one might have a real need to use them with APPLY. Not requiring

(SETF (APPLY #'AREF ...) ...) to work might be seen as an

incompatible change to the language since CLtL implies that it

is legitimate. BIT and SBIT are very similar to AREF. Requiring

other SETF places to work with APPLY might be seen as putting

unnecessary restrictions on implementors.

Current Practice:


Cost to Implementors:

Probably small. At most, you'd have to provide a new

definition for the SETF method for AREF and friends, or write some

special-case handling for these places in the SETF method for APPLY.

Cost to Users:

Probably none. Any programs that rely on using SETF of APPLY

with other standard SETF places are almost certainly

nonportable anyway.

Cost of non-adoption:

The language specification is unclear on this point.

Performance impact:

Probably none.


The language specification is made more clear.




Loosemore isn't really satisfied with proposal ONLY-AREF-AND-FRIENDS.

It is probably closest to the status quo, but it seems like an overly

complicated and messy way of dealing with a few special cases.

From a purely esthetic point of view, it would be much cleaner

to require that the SETF method for AREF be implemented by a

function (SETF AREF) and then either remove the SETF-of-APPLY

feature completely or redefine it so that

(SETF (APPLY #'<name> . <args>) <value>)

is equivalent to

(APPLY #'(SETF <name>) <value> . <args>)

except for preserving the left-to-right order of evaluation of

the original call. This kind of change might be seen as too much

of a restriction on implementors and too major to be considered

at this point, however. See issue SETF-FUNCTIONS-AGAIN.

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