Issue SETF-OF-APPLY Writeup

Issue:               SETF-OF-APPLY

References:          SETF

Related issues:      SETF-FUNCTIONS-AGAIN

Category:            CLARIFICATION

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.


Proposal (SETF-OF-APPLY:ONLY-AREF-AND-FRIENDS):


  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

  undefined.  


  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.


Rationale:


  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:


   Unknown.


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.


Benefits:


   The language specification is made more clear.

  

Esthetics:


   Ugly.


Discussion:


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.