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.