CLHS: Issue SETF-FUNCTIONS-AGAIN Writeup

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## 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:

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