CLHS: Issue DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE Writeup

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## Issue DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE Writeup

    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.

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