CLHS: Issue KEYWORD-ARGUMENT-NAME-PACKAGE Writeup

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## Issue KEYWORD-ARGUMENT-NAME-PACKAGE Writeup

    Version 6 conditionally passed X3J13/Jun87. Version 8 distributed in hardcopy
    form X3J13/Nov87.

    !
    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
                  11-Jun-87, Version 6 by Masinter
                  23-Oct-87, Version 7 by Masinter
                   8-Nov-87, Version 8 by Moon

    Problem Description:

    CLtL says that only keyword symbols can be used as keyword-names in
    &key parameter specifiers (page 60, "each -keyword- must be a
    keyword symbol, such as :start.")

    As Common Lisp is currently defined, if someone wants to define a
    function that accepts keyword (rather than positional) arguments whose
    keyword-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 the keyword-names of keyword
    parameters; allow any symbol. 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-name when matching arguments to parameter
    specifiers.  A keyword-name that is not a keyword symbol can be
    specified with the ((-keyword- -var-) ...) syntax of parameter
    specifier. The -keyword- can be any symbol, not just a keyword.

    A future specification of Common Lisp could be written with revised
    terminology that did not use the term "keyword" to refer to three
    different things: symbols in the KEYWORD package, symbols beginning
    with & that have special meaning in lambda-lists, and keyword-names
    used to match function arguments to keyword parameter specifiers.
    However, this proposal does not propose to change any terminology.

    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
    keyword-names to be symbols, and disallowing numbers, but does not
    speak to the issue of whether or not those symbols should be further
    restricted to be in the KEYWORD package.

    The desire for keyword parameters whose keyword-names are not in the
    KEYWORD package arises when the set of keyword-names accepted by a
    function is the union of the sets of keyword-names 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 keyword-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 arguments and pass
    them on to one or more applicable methods, with each method defining its
    own set of keyword-names that it is interested in.  If this proposal is
    not adopted, either the keyword-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 arguments from the
    caller and passes those arguments along to an internal routine with
    additional arguments of its own, and suppose that keyword parameters
    are used to receive these arguments.

     (IN-PACKAGE 'FOOLAND)
     (DEFUN MAKE-FOO (&REST NAME-VALUE-PAIRS &KEY &ALLOW-OTHER-KEYS)
       (APPLY #'MAKE-FOO-INTERNAL 'EXPLICIT T NAME-VALUE-PAIRS))

    This could be done without fear that the use of EXPLICIT T would
    override some argument in NAME-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:

    Some careful rewording of the existing language in CLtL is necessary in
    the standard to avoid confusion between "keyword", indicating a symbol
    in the KEYWORD package, and "keyword name", indicating a syntactic part
    of a keyword parameter specifier.  It is likely that this is best served
    by changing those instances of "keyword" to "named argument" when the
    specification is discussing the indicator which introduces an actual
    parameter in a call to a function defined with &KEY.

    The wording which refers to named arguments as being introduced by
    keyword symbols would 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 named argument name 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 the names for named
    arguments, 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.)

    Cost to implementors:

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

    Cost to users:

    None--no existing programs will stop working.

    Benefits:

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

    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
    aesthetic or less aesthetic 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:

    The cleanup committee generally supports this extension.

    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 may be mistaken.

    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 did not adopt, a proposal to
    exclude NIL as a legal indicator. It might catch some errors, but is
    otherwise an odd restriction.

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