Issue SETF-FUNCTIONS-AGAIN Writeup

Issue:              SETF-FUNCTIONS-AGAIN

References:         SETF, FBOUNDP, FDEFINITION, DEFSTRUCT

Related issues:     Issue FUNCTION-NAME

                    Issue SETF-OF-APPLY

Category:           CLARIFICATION

Edit history:       V1, 13 May 90, Sandra Loosemore


Problem description:


The addition of SETF functions to the language has left some

lingering questions:


- Whether (FBOUNDP '(SETF <name>)) is supposed to be true of <name>s 

that have SETF methods (as defined with DEFSETF or DEFINE-SETF-METHOD)

defined, or only <name>s where there are globally defined

functions named (SETF <name>).  (Issue #17 from Loosemore's

list.)


- Whether DEFSTRUCT is required to generate SETF functions, or required

to generate SETF methods, or whether this is unspecified.  (Issue

#19)


- Whether SETF places defined in the standard are required to be

implemented by SETF functions, by SETF methods, or whether this

is unspecified.  (Issue #20)


- Whether it is permissible for a <name> to have both a SETF

method defined via DEFSETF or DEFINE-SETF-METHOD, and an associated

SETF function at the same time.  



Proposal (SETF-FUNCTIONS-AGAIN:MINIMAL-CHANGES):


  (1) Clarify that (FBOUNDP '(SETF <name>)) must return true if and

  only if there is a function named (SETF <name>) defined.  It must

  return NIL if <name> has a SETF method defined but not a SETF

  function.


  (2) Clarify that it is unspecified whether DEFSTRUCT generates

  SETF methods or SETF functions for non-:READ-ONLY slot accessors.


  (3) Clarify that, unless a SETF function is explicitly documented

  in the standard, it is unspecified whether SETF places defined in

  the standard must be implemented as SETF methods or SETF functions.

  In combination with item (1), this implies that it is unspecified

  whether (SETF <name>) is FBOUNDP for these standard SETF

  places.


  (4) Clarify that it is possible, but generally not very useful, 

  to have both a SETF method and a SETF function defined for the same

  name.


Rationale:


  This proposal requires minimal changes for implementors.


Current Practice:


  Most implementations use SETF methods (and not SETF functions)

  to implement DEFSTRUCT accessors and the standard SETF places.

  In most implementations, the namespaces for SETF methods and

  SETF functions are disjoint.


Cost to Implementors:


  Probably none.


Cost to Users:


  Probably none.


Cost of non-adoption:


  Imprecision in the language specification.


Performance impact:


  Probably not significant.


Benefits:


   The language specification is made more precise.


Esthetics:


  It would probably be better to specify whether standard SETF

  places must be implemented as SETF functions or as SETF

  methods, instead of leaving it explicitly vague.  See also

  issue SETF-OF-APPLY for one particular case (AREF) where it

  seems like a good idea to specify that the SETF place must be

  implemented as a SETF function.


Discussion: