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Issue: KEYWORD-ARGUMENT-NAME-PACKAGE (Version 3)
- To: CL-Cleanup@sail.stanford.edu
- Subject: Issue: KEYWORD-ARGUMENT-NAME-PACKAGE (Version 3)
- From: David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>
- Date: Mon, 11 May 87 21:59 EDT
References: Lambda Expressions (pp60-64)
Edit history: 20-Apr-87, Version 1 by Moon
29-Apr-87, Version 2 by Pitman
11-May-87, Version 3 by Moon
Status: Revised after discussion
CLtL says that only keyword symbols can be used as non-positional argument
names in &key parameter specifiers.
Remove restrictions on the package of non-positional argument names;
allow any symbol, including NIL.
As Common Lisp is currently defined, if someone wants to define a function
that accepts named (rather than positional) arguments whose names are
symbols in packages other than the KEYWORD package, they cannot use &KEY.
Instead, they have to duplicate the &KEY mechanism using &REST, GETF,
and (if they want error checking of argument names) DO. This suggests that
the restriction of &key to only keyword symbols is arbitrary and unnecessary.
Note that the "rationale" box on p.62 of Common Lisp: the Language is an
argument in favor of requiring non-positional argument names to be symbols,
and not allowing numbers, but does not speak to the issue of whether or not
those symbols should be further restricted to be keywords.
The desire for non-positional arguments whose names are not keyword symbols
arises when the set of non-positional arguments accepted by a function is
the union of the sets of non-positional arguments accepted by several other
functions, rather than being enumerated in a single place. In this case,
it becomes desirable to use packages to prevent accidental name clashes
among non-positional argument names of different functions.
One example of a Common Lisp application that requires this capability is
the draft proposal for an object-oriented programming standard. It will
have generic functions that accept non-positional arguments and pass them on
to one or more applicable methods, with each method defining its own set of
arguments that it is interested in. If this proposal is not adopted, either
the non-positional argument names will be required to be keywords, which
will require the methods to have non-modular knowledge of each other in
order to avoid name clashes, or the methods will have to be defined with an
ad hoc mechanism that duplicates the essential functionality of &key but
removes the restriction.
A second example of a Common Lisp application that requires this capability
is private communication channels between functions. Suppose a public
routine MAKE-FOO needs to accept arbitrary keywords from the caller and
passes those keywords along to an internal routine using keywords of its
(DEFUN MAKE-FOO (&REST KEYWORD-VALUE-PAIRS &KEY &ALLOW-OTHER-KEYS)
(APPLY #'MAKE-FOO-INTERNAL 'EXPLICIT T KEYWORD-VALUE-PAIRS))
This could be done without fear that the use of EXPLICIT T would override
some keyword in keyword-value-pairs, since the only way that could happen is
if someone had done (MAKE-FOO 'FOOLAND::EXPLICIT NIL), or if the user was
programming explicitly in the FOOLAND package, either of which is an implicit
admission of willingness to violate FOOLAND's modularity.
The following outlines the changes that would have to be made to Common
Lisp: the Language if this proposal were adopted, to aid in understanding
the impact of the proposal.
Change wording which refers to non-positional arguments as being introduced
by keyword symbols to simply refer to those arguments being introduced by
symbols. For example, in the middle of p.60, the sentence:
... each -keyword- must be a keyword symbol, such as :start.
... each -keyword- must be a symbol.
Also, the word "keyword" in the first complete sentence on p.62 would
be changed to "symbol" for similar reasons.
Add extra wording on p.60 to explain that by convention keyword symbols
are normally used as non-positional argument names, and that all functions
built into the Common Lisp language follow that convention. A language
manual might or might not choose to describe the circumstances in which
it is appropriate not to follow this convention.
Add examples to illustrate this behavior. For example, on p.64 the
following examples might be added:
((lambda (a b &key ((:sea c)) d) (list a b c d)) 1 2 :sea 6)
=> (1 2 6 NIL)
((lambda (a b &key ((c c)) d) (list a b c d)) 1 2 'c 6)
=> (1 2 6 NIL)
We do not currently know of an implementation that enforces the restriction
that this proposal seeks to remove.
Some implementations have bugs that prevent NIL from working as a keyword
argument name, but allow all non-NIL symbols. (One Symbolics version that
was checked had this bug.)
No existing programs will stop working. Some implementors might have to
rearrange their error checking slightly, but it should be very easy.
Moon was under the impression that this proposal was actually adopted
around December 1985 (although no formal mechanism for adopting
proposals existed at that time), but isn't 100% sure.
This will help with the object-oriented programming standard, among other
There will probably be an argument about whether the restriction is
more esthetic or less esthetic than the freedom, but in either case
the aesthetic effect is slight.
In any case, users who do not want to use the extended functionality
can generally avoid it.
Moon generated the original version of this proposal and supports it.
He thinks that if Common Lisp truly has a restriction that only keyword
symbols can be used as keyword names in calls to functions that take
keyword arguments, it will be more difficult to come up with an
object-oriented programming standard that fits within Common Lisp.
Pitman supports this proposal.
There was some question in the committee about whether the rationale
for the proposal was believable. I hope this version of the proposal
has resolved any doubts.