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Forum:	Cleanup


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.


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.






























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.


=> T


(SYMBOL-PACKAGE *BAR-SYMBOL*) is unspecified


(PRIN1-TO-STRING *BAR-SYMBOL*) is unspecified

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
















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.


Better control of storage usage would be available portably.


No significant effect.


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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