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Issue: PACKAGE-DELETION (Version 5)
- To: CL-Cleanup@SAIL.Stanford.EDU
- Subject: Issue: PACKAGE-DELETION (Version 5)
- From: masinter.pa@Xerox.COM
- Date: 21 Nov 88 17:18 PST
- Line-fold: NO
I attempted to clarify that FIND-PACKAGE and INTERN have unspecified
results if handed a deleted-package object.
There was I think a mistake in the description of what happens if
other packages use the one being deleted, since it said something about
there being no other packages on the use list, but generally, if there is
a "use list" at all , it points to packages that are used rather than packages
that use the given one.
Can anyone supply a sample implementation of DELETE-PACKAGE?
!
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.
Test Case:
(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.