CLHS: Issue IN-PACKAGE-FUNCTIONALITY Writeup

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## Issue IN-PACKAGE-FUNCTIONALITY Writeup

    Status: 	proposal MAR89-X3J13 passed, as amended, Mar 89 X3J13
    Issue:          IN-PACKAGE-FUNCTIONALITY
    References:     IN-PACKAGE (p182-183)
    Category:       CHANGE
    Edit history:   07-Jul-88, Version 1 by Pitman
                     7-Oct-88, Version 2 by Masinter (discussion)
                     8-Dec-88, Version 3 by Masinter
                    12-Dec-88, Version 4 by Masinter
                    20-Jan-89, Version 5 by Loosemore
                    30-Jan-89, Version 6 by Loosemore
                    10-Mar-89, Version 7 by Loosemore
                    15-Mar-89, Version 8 by Masinter (add back SELECT-ONLY)
                     9-Apr-89, Verison 9 by Masinter, as amended Mar 89 X3J13,
    				and discussion updated.

    Related-Issues: DEFPACKAGE (passed)
    		COMPILE-FILE-SYMBOL-HANDLING

    Problem Description:

      There are two typical uses for IN-PACKAGE -- to define/create a package
      and to select a package. The fact that these two purposes have been
      given to the same function has led to reduced error checking.

      A more general problem is that the "Put In Seven Extremely Randoms"
      convention described in CLtL is now recognized by many people as being
      unsatisfactory for both package definition and package selection.
      The DEFPACKAGE macro provides a much cleaner mechanism for package
      definition, but there is still a need for a convenient way to select
      a package that has well-defined compilation semantics.

    Proposal (IN-PACKAGE-FUNCTIONALITY:MAR89-X3J13):

      Change IN-PACKAGE from a function to a macro:

        IN-PACKAGE name						[macro]

        This macro causes *PACKAGE* to be set to the package named NAME,
        which must be a symbol or string.  An error is signalled if the
        package does not already exist.  Everything this macro does is also
        performed at compile time if the call appears at top-level.

      Remove the second paragraph of section 11.7 in CLtL.  (This includes
      the requirement for special compile-time treatment of the various
      package functions.)

      Rationale:

        This could allow improved error checking and modularity, with only
        minimal loss of functionality.

        Making IN-PACKAGE a macro rather than a function means that there
        is no need to require COMPILE-FILE to handle it specially.  Since
        DEFPACKAGE is also defined to side-effect the compilation environment,
        there is no need to require any of the package functions to be treated
        specially by the compiler.

        The language in section 11.7 of CLtL puts the burden on
        implementations of ensuring that all symbols in a file which is
        compiled and loaded end up in the same package that they would if the
        source file were loaded interpretively.  No implementation can
        possibly meet this requirement because, in general, the runtime
        behavior of the program cannot be predicted by the compiler.

      Current Practice:

        Probably no one implements this behavior exactly since it's an
        incompatible change to CLtL.

      Cost to Implementors:

        The SELECT-PACKAGE macro can be implemented trivially by using
        EVAL-WHEN in its expansion:

        (defmacro select-package (name)
            `(eval-when (eval compile load)
    	     (setq *package*
    	           (or (find-package ',name)
    		       (error "Package ~s does not exist." ',name)))))

        The changes required to COMPILE-FILE to remove the magic treatment
        of the package functions are also likely to be small.

      Cost to Users:

        In most cases, minor syntactic changes to some files would be
        necessary.  Programmers that are now using the "Put In Seven
        Extremely Randoms" convention will probably find it straightforward
        to convert their code to do a DEFPACKAGE followed by a
        SELECT-PACKAGE.

      Cost of Non-Adoption:

        The specification of COMPILE-FILE will be much more difficult to
        understand.

        The standard will require compilers to solve the halting problem.

      Benefits:

        Modular package declarations would be encouraged and errors due
        to demand-creation of packages would be easier to detect.

        The specification of COMPILE-FILE will be simplified.

        There will be a clear statement of the requirements for program
        conformance, as relating to usage of packages.

      Aesthetics:

        The fact that IN-PACKAGE is currently ambiguous about intent (whether
        the package should exist already or not) is clearly not aesthetic.
        Removing it can't be any worse.

        The fact that the currently stated requirements for handling of
        the package functions by the compiler are not implementable is
        clearly not aesthetic.
    Discussion:

      The dual use of IN-PACKAGE has not been helpful and is confusing.

      This is an incompatible change.

    KMP's notes of the Mar 89 X3J13:

     On Tuesday, we took a straw poll.
       0 opposed both proposals.
      15 liked NEW-MACRO.
       7 liked SELECT-ONLY.
     ``Keeping IN-PACKAGE makes no difference to compatibility.'' --Moon
     Pitman moved to amend this to say "deprecate" instead of remove.
     The motion to amend failed 3-N.
     The NEW-MACRO proposal passed unamended 12-4.

     On Thursday, Aaron Larson and JonL asked that the issue be reconsidered.
     The motion to reconsider passed N-1.
     There was a motion to rename the SELECT-PACKAGE which we'd voted in to
     IN-PACKAGE -- so that the compatible syntax (IN-PACKAGE "FOO") would work
     in CLtL and ANSI CL.
     Steele requested verbal clarification that we were not trying to solve
     the ``dusty file'' problem but rather to make it possible to write new code
     that worked in old and new situations -- it was agreed that this was a
     correct characterization of the proposal.
     JonL's amendment passed 13-1.
     Then the amended proposal was voted in 14-0.

    The net effect is that NEW-MACRO passed with the name of SELECT-PACKAGE
    changed to IN-PACKAGE.

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