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## Issue PACKAGE-DELETION Writeup

    Forum:	Cleanup
    Issue:        PACKAGE-DELETION
    References:   Packages (pp171-192), PACKAGE-NAME (p184), PACKAGEP (p76)
    Category:     ADDITION
    Edit history: 30-Sep-88, Version 1 by Pitman
    	      01-Oct-88, Version 2 by Pitman
    	      04-Oct-88, Version 3 by Pitman
    		(provide for correctable errors in some cases)
    	      07-Oct-88, Version 4 by JonL
    		21-Nov-88, Version 5 by Masinter

    Problem Description:

      There is no way to get rid of a package in Common Lisp.

      This absence makes interactive development work tricky in some
      implementations. If a package is accidentally built incorrectly, the
      user must either rename the package to another package or start over
      by reloading his program in a fresh lisp image.

      Some programs need to create and destroy packages at runtime.
      Without such a facility, some clumsy combination of RENAME-PACKAGE,
      UNINTERN, and UNUSE-PACKAGE is usually made to work. However, it is
      easy for a casual programmer to forget to undo some of the
      bookkeeping, leading to unwanted effects.

    Proposal (PACKAGE-DELETION:NEW-FUNCTION):

      Introduce the function DELETE-PACKAGE, described as follows:

      DELETE-PACKAGE package                                 [Function]

       Deletes PACKAGE from all package system data structures. PACKAGE may
       be either a package or the name of a package.

       If PACKAGE is a package name (i.e., not type PACKAGE) which does not
       currently name a package, a correctable error is signalled. If
       continued, no deletion action is attempted. Instead, DELETE-PACKAGE
       immediately returns NIL.

       If PACKAGE is a package object (i.e., an object of type PACKAGE)
       which has already been deleted, no error is signalled and no further
       deletion action is attempted. Instead, DELETE-PACKAGE immediately
       returns NIL.

       If the designated package is used by other packages, a correctable
       error is signalled. If continued, the effect of UNUSE-PACKAGE is
       done to remove any dependencies, causing its external symbols to stop
       being accessible to those packages. Once this is done, DELETE-PACKAGE
       goes on to delete the package just as it would had there been no
       packages that used it.

       After this operation completes, the contents of the symbol-package
       slot of any symbol homed in the deleted package is unspecified; for
       those symbols not homed in that package, the contents remain unchanged.
       Except for this, symbols in the deleted package are not modified in
       any other way.

       The effect of DELETE-PACKAGE is that the name and
       nicknames of the designated package cease to be recognized package
       names.   The package object is still a package -- PACKAGEP is true
       of it -- but  PACKAGE-NAME will return NIL.  The effect of any other
       package operation on PACKAGE once it has been deleted is undefined.
       In particular, FIND-SYMBOL, INTERN and other functions that
      look for a symbol name in a package will have unspecified results if
      called with *PACKAGE* bound to the deleted package or with the
      deleted package as an argument.

       DELETE-PACKAGE returns T if the deletion attempt was successful
       and NIL otherwise.

    Examples:

      (SETQ *FOO-PACKAGE* (MAKE-PACKAGE "FOO" :USE NIL))
      (SETQ *FOO-SYMBOL*  (INTERN "FOO" *FOO-PACKAGE*))
      (EXPORT *FOO-SYMBOL* *FOO-PACKAGE*)

      (SETQ *BAR-PACKAGE* (MAKE-PACKAGE "BAR" :USE '("FOO")))
      (SETQ *BAR-SYMBOL*  (INTERN "BAR" *BAR-PACKAGE*))
      (EXPORT *FOO-SYMBOL* *BAR-PACKAGE*)
      (EXPORT *BAR-SYMBOL* *BAR-PACKAGE*)

      (SETQ *BAZ-PACKAGE* (MAKE-PACKAGE "BAZ" :USE '("BAR")))

      (SYMBOL-PACKAGE *FOO-SYMBOL*)        => #<Package "FOO">
      (SYMBOL-PACKAGE *BAR-SYMBOL*)        => #<Package "BAR">

      (PRIN1-TO-STRING *FOO-SYMBOL*)       => "FOO:FOO"
      (PRIN1-TO-STRING *BAR-SYMBOL*)       => "BAR:BAR"

      (FIND-SYMBOL "FOO" *BAR-PACKAGE*)    => FOO:FOO, :EXTERNAL

      (FIND-SYMBOL "FOO" *BAZ-PACKAGE*)    => FOO:FOO, :INHERITED
      (FIND-SYMBOL "BAR" *BAZ-PACKAGE*)    => BAR:BAR, :INHERITED

      (PACKAGEP *FOO-PACKAGE*)             => T
      (PACKAGEP *BAR-PACKAGE*)             => T
      (PACKAGEP *BAZ-PACKAGE*)             => T

      (PACKAGE-NAME *FOO-PACKAGE*)         => "FOO"
      (PACKAGE-NAME *BAR-PACKAGE*)         => "BAR"
      (PACKAGE-NAME *BAZ-PACKAGE*)         => "BAZ"

      (PACKAGE-USE-LIST *FOO-PACKAGE*)     => ()
      (PACKAGE-USE-LIST *BAR-PACKAGE*)     => (#<Package FOO>)
      (PACKAGE-USE-LIST *BAZ-PACKAGE*)     => (#<Package BAR>)

      (PACKAGE-USED-BY-LIST *FOO-PACKAGE*) => (#<Package BAR>)
      (PACKAGE-USED-BY-LIST *BAR-PACKAGE*) => (#<Package BAZ>)
      (PACKAGE-USED-BY-LIST *BAZ-PACKAGE*) => ()

      (DELETE-PACKAGE *BAR-PACKAGE*)
      Error: Package BAZ uses package BAR.
      If continued, BAZ will be made to unuse-package BAR,
    	        and then BAR will be deleted.
      Type :CONTINUE to continue.
      Debug> :CONTINUE
      				       => T

      (SYMBOL-PACKAGE *FOO-SYMBOL*)        => #<Package "FOO">
      (SYMBOL-PACKAGE *BAR-SYMBOL*)        is unspecified

      (PRIN1-TO-STRING *FOO-SYMBOL*)       => "FOO:FOO"
      (PRIN1-TO-STRING *BAR-SYMBOL*)       is unspecified

      (FIND-SYMBOL "FOO" *BAR-PACKAGE*)    is undefined

      (FIND-SYMBOL "FOO" *BAZ-PACKAGE*)    => NIL, NIL
      (FIND-SYMBOL "BAR" *BAZ-PACKAGE*)    => NIL, NIL

      (PACKAGEP *FOO-PACKAGE*)             => T
      (PACKAGEP *BAR-PACKAGE*)             => T
      (PACKAGEP *BAZ-PACKAGE*)             => T

      (PACKAGE-NAME *FOO-PACKAGE*)         => "FOO"
      (PACKAGE-NAME *BAR-PACKAGE*)         => NIL
      (PACKAGE-NAME *BAZ-PACKAGE*)         => "BAZ"

      (PACKAGE-USE-LIST *FOO-PACKAGE*)     => ()
      (PACKAGE-USE-LIST *BAR-PACKAGE*)     is undefined
      (PACKAGE-USE-LIST *BAZ-PACKAGE*)     => ()

      (PACKAGE-USED-BY-LIST *FOO-PACKAGE*) => ()
      (PACKAGE-USED-BY-LIST *BAR-PACKAGE*) is undefined
      (PACKAGE-USED-BY-LIST *BAZ-PACKAGE*) => ()

    Rationale:

      This facility corrects the deficiency described in the problem description.

    Current Practice:

      Symbolics has a function PKG-KILL which satisfies the proposed behavior
      except that an error is not signalled if the package is used.
      When a package is killed by PKG-KILL, the home package of all symbols
      in that package are left undisturbed (i.e., local symbols pointing to
      the killed package); this aspect is compatible with the stated proposal.

      Procyon Common Lisp has a function called DELETE-PACKAGE already. It
      returns the name   of the package so deleted (as a string). [Perhaps it also
      differs in the correctability of the errors it signals?]

      Lucid Common Lisp implements DELETE-PACKAGE, except that the continuation
      option for a name that doesn't name a package is different.

    Cost to Implementors:

      The cost of providing this facility is probably small.

    Cost to Users:

      Very slight to none. This change is essentially compatible.

      Some code which cached packages in variables might have to be slightly
      more cautious, but experience in the Symbolics implementation suggests
      that it's really the responsibility of the person doing the DELETE-PACKAGE
      to take care of worrying about the effects of having deleted the package:
      normal programs need not bother testing a package for validity (using
      PACKAGE-NAME) before using it.

    Cost of Non-Adoption:

      Getting rid of a package would continue to be difficult to do portably.

    Benefits:

      Better control of storage usage would be available portably.

    Aesthetics:

      No significant effect.

    Discussion:

      This was discussed as part of a larger bulk issue of how to undo all
      sorts of definitions. Since that proposal has not gone anywhere
      (perhaps bogged down under its own weight), this subtopic has been
      broken off for separate discussion.

      Note that if a symbol's package component is modified as a result
      of being "unintern'd" from a delete packaged, then it is unspecified
      as to how it will  be printed.

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