CLHS: Issue ASSOC-RASSOC-IF-KEY Writeup

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## Issue ASSOC-RASSOC-IF-KEY Writeup

    Issue:        ASSOC-RASSOC-IF-KEY
    References:   ASSOC-IF (p280), ASSOC-IF-NOT (p280), RASSOC-IF (p281),
                  RASSOC-IF-NOT (p281)
    Category:     ENHANCEMENT
    Edit history: 22-Apr-87, Version 1 by Pitman
                  20-Nov-87, Versions 2,3 by Masinter
                  23-Nov-87, Version 4 by Masinter

    Problem Description:

    The descriptions of ASSOC-IF, ASSOC-IF-NOT, RASSOC-IF, and RASSOC-IF-NOT do not
    mention a :KEY option, although ASSOC and RASSOC have one.

    This is often reported as an inconsistency in Common Lisp.

    Proposal (ASSOC-RASSOC-IF-KEY:YES):

    Allow a :KEY keyword for ASSOC-IF, ASSOC-IF-NOT, RASSOC-IF, and RASSOC-IF-NOT.
    If not supplied, it should default to #'IDENTITY as do the :KEY keywords for
    other -IF and -IF-NOT functions. The function, as with the :KEY argument for
    ASSOC and RASSOC, is applied to the CAR of the pair in the association list for
    ASSOC-IF and ASSOC-IF-NOT and the CDR of the pair for RASSOC-IF and
    RASSOC-IF-NOT.

    Documentation impact:

    A better description of the intent might be to say that the car /contains/ the
    key of the association, and by default the car /is/ the key of the association.

    Example:

    (assoc-if #'zerop pathnames :key #'pathname-version)

    could be used to search a list indexed by pathnames finding one with zero
    version.

    Rationale:

    This is an inconsistency in the language that is simple to fix.

    Current Practice:

    Symbolics implements :KEY for the -IF and -IF-NOT assoc functions. Others follow
    the book and allow :KEY only for ASSOC.

    Cost to Common Lisp implementors:

    A small amount of additional code is necessary to support this in
    implementations not already offering it as an extension.

    Cost to Common Lisp users:

    The change is essentially upward compatible with user code.

    Benefits:

    This would make the set of -IF and -IF-NOT functions be more regular in their
    calling conventions.

    Aesthetics:

    All the other -IF and -IF-NOT variations of list operations omit the :TEST and
    :TEST-NOT keywords, but allow :KEY. For example, consider the family of MEMBER,
    MEMBER-IF, and MEMBER-IF-NOT. Although this introduces additional mechanism, it
    does so in a way that probably makes it easier to think about which functions do
    what, so it would likely be seen as a simplification.

    Discussion:

    The omission of :KEY in this situation in CLtL was probably an oversight.

    The cleanup committee supports this proposal.

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