CLHS: Issue DEPRECATION-POSITION Writeup

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## Issue DEPRECATION-POSITION Writeup

    Issue:        DEPRECATION-POSITION
    References:   X3J13 committee and sub-committee meetings
    Category:     Policy
    Edit history: 12-DEC-88, Version 1 by Chapman
                  20-DEC-88, Version 2 by Chapman
                  9-JAN-89, Version 3 by Chapman

    Problem Description:

    When features of a language become outdated due to technology advances,
    or unnecessary due to the addition of better features, should the old
    features be deprecated from the language, or deleted outright?

    Since there has never been a Common Lisp standard before, deprecation
    is against a de-facto standard, which may or may not be appropriate.
    Deletion, on the other hand, is cleaner, but may generate never-ending
    discussion when the standard goes for public review (and even in the
    X3J13 meeting).

    In summary, there are two levels:
    1) features in CLtL that are not present in ANSI Common Lisp 1989,
    2) features in ANSI Common Lisp 1989 that will likely not be present in
    future standards;

    and the issues are:
    a) what features fit into which category
    b) how should implementations deal with such features? how can programs be
    written to avoid problems with such features?

    Proposal (DEPRECATION-POSITION:LIMITED)

    Since Common Lisp has been used industrially, it is reasonable to
    assume that any level 1 feature will be a cause for
    at least some controversy. Therefore, this proposal suggests that
    X3J13 put features categorized as level 1 in a separate package,
    and retain features categorized as level 2 in the Lisp package, but
    declare them deprecated in the standard.
    The members of each level will be determined on a case-by-case basis by
    the X3J13 committee.

    Rationale:

    The standard should contain information about deprecation since
    the topic has been mentioned more than once in X3J13, and since
    Common Lisp is such a large language.

    It's not
    unreasonable for a language the size of Lisp to get rid of the
    never-used tools, but we don't want to generate years of discussion
    over a relatively minor issue when the standard goes for public review.

    Current Practice:

    Fortran successfully deprecated one constant. However, a proposal
    submitted during its latest standardization effort that
    suggested deleting old features in favor of new ones was
    opposed vehemently. The Pascal committee is currently opposed
    to deprecation, and the SPARC proposal suggests that
    deprecation should be dictated by the marketplace.

    Adoption Cost:

    None.

    Benefits:

    This policy will provide a basis for future X3J13 decisions.

    Conversion Cost:

    None.

    Aesthetics:

    None.

    Discussion:

    Comment:
    "I personally would argue for not including "deprecated" features in
    the standard at all, because including them now will make it harder to
    remove them later.  My perception is that ANSI Common Lisp is turning
    out to be a much different language than what is described in CLtL,
    particularly if CLOS becomes a required part of the language.  If,
    with the benefit of hindsight and experience, we now realize that some
    "features" described in CLtL were really "bugs", the time to remove
    them is *before* they become cast in stone as part of ANSI Common
    Lisp.  I suspect that most implementors will continue to provide these
    features as extensions anyway, as long as a substantial number of
    their customers are still using them."

    Response:
    Perhaps most implementors will continue to provide the deleted features,
    but they will do that also if they are deprecated. Since the only real
    difference between the two is the amount of discussion one will generate
    over the other (I think that worrying about future difficulty of getting
    rid of the features is not a "real" difference yet), it seems that choosing
    the route of least resistance in this case is the wisest course.

    Comment:
    "For the most part, X3J13 hasn't been able to deal with
    which features fit into which category until how the categories will
    be divided has been resolved. In particular, there are a number of
    features that we didn't want to remove from ANSI Common Lisp 1989 if
    it would be awkward for implementations to continue to support them or
    programs to continue to use them, but wanted to at least declare them
    "obsolete". There's been some debate on whether CHAR-FONT, for
    example , should be deleted before the standard is written, or declared
    deprecated within the standard."

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