CLHS: Issue FORMAT-STRING-ARGUMENTS Writeup

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## Issue FORMAT-STRING-ARGUMENTS Writeup

    Issue:               FORMAT-STRING-ARGUMENTS
    References:
    Related issues:      Issue PRETTY-PRINT-INTERFACE
    Category:            CLARIFICATION, CHANGE
    Edit history:        V1, 10 May 90, Sandra Loosemore

    Problem description:

    Issue PRETTY-PRINT-INTERFACE says that
        The function FORMAT is extended so that it can accept a
        function instead of a FORMAT string.  (This change is also made
        in the other functions that accept FORMAT strings such as ERROR
        and WARN.)

    The pretty printer proposal did not explicitly address whether
    this also affects the :FORMAT-STRING slot of simple-condition
    objects, and whether any macros (as well as functions) specified
    in the standard were affected by the change.

    Also, there is potential for confusion in using the terminology
    "format strings" to refer to objects that are now permitted to
    be either strings or functions.

    There are two proposals, SPECIFY and RETRACT.

    This is issue #1 from Loosemore's list.

    Proposal (FORMAT-STRING-ARGUMENTS:SPECIFY):

      (1) Clarify that the following functions and macros which formerly were
      specified to take format strings as arguments now accept either
      a format string or a format function:

      ASSERT (datum; treated like a string argument is now)
      BREAK (format-string)
      CERROR (both continue-format-string and datum arguments)
      DEFINE-METHOD-COMBINATION (:DESCRIPTION argument)
      ERROR (datum; treated like a string argument is now)
      FORMAT (control-string)
      INVALID-METHOD-ERROR  (format-string)
      METHOD-COMBINATION-ERROR (format-string)
      SIGNAL (datum; treated like a string argument is now)
      WARN (datum; treated like a string argument is now)
      WITH-SIMPLE-RESTART (format-string)
      Y-OR-N-P (format-string)
      YES-OR-NO-P (format-string)

      (2) Clarify that the :FORMAT-STRING argument passed to MAKE-CONDITION
      to construct a condition which is a subtype of SIMPLE-CONDITION
      may be either a string or a function, and the accessor
      SIMPLE-CONDITION-FORMAT-STRING can return either a string or a function.

      (3) Change the name of the :FORMAT-STRING argument to MAKE-CONDITION
      associated with SIMPLE-CONDITION types to :FORMAT-CONTROL, and
      rename the accessor function to SIMPLE-CONDITION-FORMAT-CONTROL.  As
      an editorial matter, change all references to arguments now
      named "format-string" to "format-control".

      Rationale for proposal SPECIFY:

      Items (1) and (2) were probably what was intended by the
      PRETTY-PRINT-INTERFACE proposal.  Item (3) is a logical
      extension; since the arguments need not be strings, calling
      them "format-strings" is a misnomer.

    Proposal (FORMAT-STRING-ARGUMENTS:RETRACT):

      (1) Retract the part of issue PRETTY-PRINT-INTERFACE that required
      FORMAT, related functions such as ERROR and WARN, and the ~?
      and ~{~} directives to accept format control functions as well
      as strings.

      (2) Remove the FORMATTER macro from the language.

      Rationale for proposal RETRACT:

      It is easier to remove this feature than to try to fix the rest
      of the language to be consistent with it.  Having a macro to
      "compile" format strings into functions is probably not useful
      in the absence of the extensions to FORMAT.

    Current Practice:

      Unknown.  The pretty printer specification is a fairly recent
      addition to the language and it probably hasn't been fully
      integrated into any implementation yet.

    Cost to Implementors:

      The actual implementation cost of extending the listed functions
      and macros to accept function arguments is probably small, since
      in most cases they just pass the argument to FORMAT anyway.
      Both proposals probably involve changes to documentation.

    Cost to Users:

      For proposal SPECIFY, the incompatible change to the name of the
      :FORMAT-STRING slot of SIMPLE-CONDITION objects may cause some
      problems for users who have started using the condition system
      in their code, but these ought to be fairly straightforward to
      track down and fix.

      Proposal RETRACT may cause some problems for users who have
      started writing code which uses the FORMATTER macro and the
      extension to FORMAT to accept functions, but again the
      problems should generally be easy to find and fix.

    Cost of non-adoption:

      Parts of the language are poorly specified.

    Performance impact:

      The ability to "compile" format strings into functions in advance can
      potentially lead to greater runtime efficiency.  On the other
      hand, some implementations might implement FORMATTER in such a
      way that it is less efficient than passing a format string
      directly.  Furthermore, even without FORMATTER in the language,
      it is still possible for implementations to do a compile-time
      transformation to "compile" constant string arguments to FORMAT
      and related functions.

    Benefits:

      The language is better specified.

    Esthetics:

      Either proposal would be an improvement over the current
      situation.

    Discussion:

      Conceivably, the FORMATTER macro could be left in the language
      even while removing the extensions to FORMAT and friends, but it's
      not clear how useful this would be to users.

      Loosemore opposes other partial solutions (like requiring FORMAT
      to accept both strings and functions but specifying everything else
      to accept only strings) on the grounds that they would
      introduce additional, unnecessary complexity and inconsistency
      into the language.  If the feeling is that this feature belongs
      in the language, we ought to bite the bullet and do it
      properly.

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