Issue: KEYWORD-ARGUMENT-NAME-PACKAGE (Version 5)

[Masinter.pa@Xerox.COM]

I made the discussion less wishy-washy by saying we rejected disallowing
NIL.  I made some edits to fix typos various people pointed out. I fixed
the "grinding".

Status: Ready for release.

!
Issue:        KEYWORD-ARGUMENT-NAME-PACKAGE
References:   Lambda Expressions (CLtL pp60-64)
Category:     CLARIFICATION/CHANGE
Edit history: 20-Apr-87, Version 1 by Moon
	         29-Apr-87, Version 2 by Pitman
              11-May-87, Version 3 by Moon
              29-May-87, Version 4 by Masinter
               5-Jun-87, Version 5 by Masinter

Problem Description:

CLtL says that only keyword symbols can be used as non-positional
argument names in &key parameter specifiers.

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.  

Some applications (including the draft proposal for the Common Lisp
Object System (CLOS)) require this capability. [See Rationale below.]

 
Proposal (KEYWORD-ARGUMENT-NAME-PACKAGE:ANY)

Remove restrictions on the package of non-positional argument names;
allow any symbol, including NIL.  That is: 

If, following an &key, a variable appears alone or in a (variable
default-value) pair, the behavior specified in CLtL is unchanged: a
keyword-symbol with the same print name as the variable is created and
is used as the keyword-indicator in function calls.  The only way to get
a non-positional-argument-name that is not a keyword symbol is to use
the (indicator variable) syntax in the function's lambda list.  The
keyword-indicator can be any symbol, not just a keyword.

Test case:

(DEFUN RESULT (&KEY ((SECRET-KEYWORD SECRET) NIL) AMOUNT)
    (FORMAT NIL "You ~A $~D" (if SECRET "win" "lose") AMOUNT))

(RESULT :AMOUNT 100) => "You lose $100"
(RESULT :AMOUNT 100 'SECRET-KEYWORD T) => "You win $100"


Rationale:

The "rationale" box on p.62 of CLtL 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
(CLOS).  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 own.
 (IN-PACKAGE 'FOOLAND)
 (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.

Documentation Impact:

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.

The wording which refers to non-positional arguments as being introduced
by keyword symbols wuuld change 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.
 would become
   ... each -keyword- must be a symbol.

The word "keyword" in the first complete sentence on p.62 would be
changed to "symbol" for similar reasons.

Extra wording would have to be added 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.

Examples would be useful. 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)

Current Practice:

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.)

Adoption Cost:

Some implementors might have to rearrange their error checking slightly,
but it should be very easy.

Benefits:

This will help with the object-oriented programming standard, among
other things.

Conversion Cost:

None--no existing programs will stop working.  

Aesthetics:

The restriction of &key to only keyword symbols is arbitrary and
unnecessary.

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.

Discussion:

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.

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.

The cleanup committee considered but rejected a proposal to exclude NIL
as a legal indicator. It might catch some errors, but is otherwise an
odd restriction.

The cleanup committee supports this change.