Related issues: SETF-FUNCTIONS-AGAIN
Edit history: V1, 10 May 90, Sandra Loosemore
V2, 29 May 90, Sandra Loosemore (forgot BIT & SBIT)
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.
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
Cost of non-adoption:
The language specification is unclear on this point.
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.