Issue: SHADOW-ALREADY-PRESENT (version 2)

[David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>]

Issue:         SHADOW-ALREADY-PRESENT

References:    CLtL p.186

Category:      CLARIFICATION

Edit history:  Moon 24 Aug 87 new issue (version 1)
               Moon 27 Aug 87 incorporate suggestions from JonL (version 2)

Problem description:

The description of the SHADOW function can be interpreted as saying that
the function has no effect, if the symbol is already present in the
package.  This happens if the third sentence in the description ("then
nothing is done") is interpreted as applying to the entire description
rather than just to the fourth sentence.

SHADOW is said to take symbols as arguments, however only the print-name
is meaningful for this operation (that fact is already documented).

Proposal SHADOW-ALREADY-PRESENT:WORKS:

The SHADOW function always adds the symbol to the PACKAGE-SHADOWING-SYMBOLS
list, even when the symbol is already present in the package.  In addition,
the first argument is allowed to be or contain strings as well as symbols.
The specification "the print name of each symbol is extracted" is to be
modified accordingly.

Test Case:

(make-package 'test-1)
(intern "TEST" (find-package 'test-1))
(shadow 'test-1::test (find-package 'test-1))
(assert (not (null (member 'test-1::test (package-shadowing-symbols
                                           (find-package 'test-1))))))
(make-package 'test-2)
(intern "TEST" (find-package 'test-2))
(use-package 'test-2 (find-package 'test-1))    ;should not error

To test the use of strings in place of symbols, change the
third line of the test case to
(shadow "TEST" (find-package 'test-1))
Note the use of capital letters in the string.

Rationale:

Page 180 describes a name conflict problem that can occur when calling
the function USE-PACKAGE. The name conflict is between a symbol directly
present in the using package and an external symbol of the used package.
This name conflict may be resolved in favor of the symbol directly
present in the using package by making it a shadowing symbol.  For this
to work, SHADOW must add the symbol to the PACKAGE-SHADOWING-SYMBOLS list
even when it is already present in the package.

Since only the print name of a symbol argument is meaningful, a string
should also be accepted.  This is particularly useful to avoid problems
when compiling code in one package environment and loading it into a
slightly different package environment, where the symbol that was referred
to at compile time may not be present at load time.  This is particularly
important because the symbol referred to by the print name may be changed
by evaluation of the SHADOW form.  A close reading of CLtL shows that one
can already use (shadow '#:bar) in place of (shadow 'bar), to achieve much
the same effect as (shadow "BAR").  But the user should not have to play
such games, strings should be accepted.

Current practice:

[I have not confirmed any of these myself except Symbolics --Moon]

Symbolics and Spice Lisp add the symbol to the PACKAGE-SHADOWING-SYMBOLS
list, even when the symbol is already present in the package.  Kyoto Common
Lisp, Lucid Common Lisp, and Xerox Common Lisp ignore SHADOW when the
symbol is already present in the package.  It seems likely that we will
find several implementations in each camp.

SHADOW accepts strings in Symbolics Common Lisp.

Adoption Cost:

It should be two one-line changes in one function.

Cost of non-adoption:

Inconsistency among implementations and lack of a clear way to resolve the
name conflict mentioned in Rationale.

Benefits:

Consistency among implementations and fewer mysterious package problems.

Conversion Cost:

Technically this would be an incompatible change to implementations that do
not already behave as proposed, however it is difficult to conceive of a
user program that would require any conversion to cope with the change.
Some users might want to remove kludges that were only necessary to get
around the former misbehavior of SHADOW.

Esthetics:

The proposal would remove an unnecessary special case, thus simplifying the
language slightly.

Discussion:

The issue was raised by Dieter Kolb on the Common-Lisp mailing list.

It would be useless for SHADOW to fail to put a symbol that is already
present in the package onto the PACKAGE-SHADOWING-SYMBOLS list.  Moon
believes CLtL intended to describe what is being proposed, but
unfortunately used ambiguous language.

Epigram:

"Who knows what evil lurks in the hearts of men?"