Issue: DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE (Version 3)

[masinter.pa@Xerox.COM]

I shouldn't work on these things late at night....

The discussion from Kim Barrett mentioned "supplied-p" arguments.
I tried to think of uses of supplied-p arguments, and I thought
they were most useful in helping with some of the other
initialization variables. So must supplied-p arguments name
slots? If not, are arguments allowed which don't name slots
but are only used in the computation of &AUX or other 
&OPTIONAL or &KEY defaults? I'm guessing so and propose
this amendment which enhances the proposal.

Let me know if you think this is a bad idea.

!
Forum:         Cleanup
Issue:         DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE
References:    CLtL page 316
Category:      CHANGE
Edit history:  20-Sep-88, Version 1, Peck
               21-Sep-88, Version 2, Masinter, minor revisions
                8-Jan-89, Version 3, Masinter


Problem description:

Currently, DEFSTRUCT constructor functions can be either the default
constructor function, with *only* keyword arguments, or it can be a 
so-called "By Order of Arguments" constructor function with explicitly
*no* keyword arguments.  Other functions in Common Lisp allow a free
mix of required, optional, and keyword arguments. 

With the current restriction, it is necessary to hand code a function that
will accept optional and keyword arguments and parse the supplied-p
variables explicitly.  Even so, it is not obvious to the casual programmer
how to provide the same semantics as defstruct does with respect to default
values and the defstruct init-forms.

Proposal: DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE:ALLOW-KEY

Allow &KEY keyword arguments in constructor forms of DEFSTRUCTs
and the &ALLOW-OTHER-KEYS token in addition to the &OPTIONAL,
&REST and &AUX arguments already allowed. Keyword arguments default
in a manner similar to that of &OPTIONAL arguments: if no default
is supplied in the lambda-list then the slot initform is used;
otherwise the slot is not initialized -- its initial value is
undefined.

If keyword arguments of the form ((key var) [default [svar]])
are specified, the "slot name" is matched with VAR (and not KEY).

Additional arguments that do not correspond to slot names but
are merely present to supply values used in subsequent initialization 
computations are allowed.


Examples:

It should be possible to write forms like this:

(defstruct (foo (:constructor CREATE-FOO (a &optional b (c 'sea)
					    &key (d 2)
					    &aux e (f 'eff))))
  (a 1) (b 2) (c 3) (d 4) (e 5) (f 6))

(create-foo 10) => #S(foo a 10 b 2 c sea d 2 e nil f eff)
(create-foo 10 'bee 'see :d 'dee) => #S(foo a 10 b bee c see d dee e nil f eff)

In the definition:
(defstruct (frob (:constructor create-frob
		(a &key (b 3 have-b) (c-token 'c) 
		        (c (list c-token (if have-b 7 2))))))
	a b c)

the c-token argument is used merely to supply a value used in the 
initialization of the c slot. The "supplied-p" arguments of
keyword arguments might be of this form.

Rationale:

This is a logical extension of the specification which makes some
programming easier.

Current practice:

Many implementations signal an error if given &KEY arguments or
arguments that are not slot names. The latest version of IIM Common 
Lisp allows &KEY arguments in this manner. Envos Medley
(Xerox Common Lisp) implements the proposal. 

Cost to Implementors:

The modifications to allow intermixed keywords and optionals in implementations
that don't already are likely simple. 

Cost to Users:

No cost, this is upward compatible.

Cost of non-adoption:

The current situation is non-intuitive and needless restrictive.

Benefits:

Much easier for users to write the constructor function they want.
Probably implementation code would be reduced, since this would no 
longer be an error.

Esthetics:

Minor improvement since it removes a needless restriction.

Discussion:

Possibly  references to "By-position", "positional", and "By Order of
Arguments" constructor function might need to be changed to something else in
the standard.  (They can still be called BOA-constructors, though, right?  :-)

Version 2 of this proposal was on the January 1989 ballot.