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.