CLHS: Issue CONSTANTP-ENVIRONMENT Writeup

#

## Issue CONSTANTP-ENVIRONMENT Writeup

    Issue:               CONSTANTP-ENVIRONMENT
    References:          CONSTANTP
    Related issues:      COMPILE-FILE-HANDLING-OF-TOP-LEVEL-FORMS
                         DEFCONSTANT-SPECIAL
                         SUBTYPEP-ENVIRONMENT
                         SYMBOL-MACROS-AND-PROCLAIMED-SPECIALS
                         CONSTANTP-DEFINITION
    Category:            CHANGE, ADDITION
    Edit history:        v1, 12 Feb 1991, Sandra Loosemore
                         v2, 15 Feb 1991, Sandra Loosemore (comments from KAB)
                         v3, 13 Mar 1991, Sandra Loosemore

    Problem description:

      It is permitted for DEFCONSTANT to note the definition of the constant
      at compile-time in such a way that the definition is visible only to
      the file compiler and not to the evaluator.  This can lead to
      incorrect behavior in user code that uses CONSTANTP, for example in
      macro expansion functions.  Such code needs to explicitly indicate in
      some way whether it wishes to deal with constant definitions in the
      environment seen by the file compiler (the "remote environment"), or
      in the current environment.

    Proposal (CONSTANTP-ENVIRONMENT:ADD-ARG):

      Add an optional environment argument to the function CONSTANTP.  If
      the argument is NIL or not supplied, it defaults to the null lexical
      and the current global environment.

      CONSTANTP may use the environment to determine whether a symbol has
      been globally defined as a constant (as with DEFCONSTANT).

      Unless at least one of proposals CONSTANTP-DEFINITION:INTENTIONAL,
      CONSTANTP-DEFINITION:ADD-ARGUMENT, or
      SYMBOL-MACROS-AND-PROCLAIMED-SPECIALS:SHADOWING-PERMITTED is accepted,
      the value returned by CONSTANTP may not depend on lexical information
      in the environment.

    Examples:

       ???

    Rationale:

      It solves the problem in a way that is consistent with the way it is
      handled in other places in the language.

    Current Practice:

      Chestnut's Lisp-to-C translator implements this proposal.

    Cost to Implementors:

      The cost of adding the environment argument is quite small.
      In implementations that do not support remote environments, the argument
      can simply be ignored.

    Cost to Users:

      The behavior of existing programs that call CONSTANTP without an
      environment argument will not be affected.  However, there is some
      motivation for users to change calls to CONSTANTP to pass an environment
      argument where appropriate, since in implementations that do support
      remote environments, calling CONSTANTP without an environment argument
      might result in NIL being returned for some things that could otherwise
      have been detected as constant.

    Cost of non-adoption:

      Some programs will work incorrectly.

    Performance impact:

      Minor.

    Benefits:

      The cost of non-adoption is avoided.

    Esthetics:

      Looks OK to me.

    Discussion:

      If proposal SYMBOL-MACROS-AND-PROCLAIMED-SPECIALS:SHADOWING-PERMITTED
      is accepted, CONSTANTP needs to accept an environment argument so that
      it can detect when a defined constant has been lexically shadowed by a
      symbol macro.

      If proposal CONSTANTP-DEFINITION:INTENTIONAL or ADD-ARGUMENT passes,
      CONSTANTP needs to accept an environment argument so that it can perform
      the additional kinds of analysis these proposals permit (such as
      macroexpansion) in the correct lexical environment.

      Proposal CONSTANTP-ENVIRONMENT:ADD-ARG only extends the syntax of
      CONSTANTP and permits CONSTANTP to use *global* information in the
      environment.  Issues SYMBOL-MACROS-AND-PROCLAIMED-SPECIALS and
      CONSTANTP-DEFINITION include proposals that would require that
      CONSTANTP also be sensitive to *lexical* information in the environment.
      The costs to users and implementors that this would involve are
      potentially much higher, but are more properly costs associated with
      those other proposals, not CONSTANTP-ENVIRONMENT:ADD-ARG.

Copyright 1996-2001, Xanalys Inc. All rights reserved.