CLHS: Issue DEFVAR-INITIALIZATION Writeup

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## Issue DEFVAR-INITIALIZATION Writeup

    Issue:        DEFVAR-INITIALIZATION
    References:   DEFVAR (p68)
    Category:     CLARIFICATION/CHANGE
    Edit history: Version 1 by Pitman 02/26/87
                  Version 2 by cleanup committee 15-Mar-87 14:45:18
                  Version 3 by Masinter (format) 15-Mar-87 18:34:28
                  Version 4 by Masinter  5-Jun-87

    Problem description:

    The description of DEFVAR on p.68 is not adequately clear on what
    happens if an initialization value is not provided, as in (DEFVAR FOO).
    Does this initialize the variable?

    Proposal (DEFVAR-INITIALIZATION:CONSERVATIVE):

    If the one-argument form of DEFVAR is used, the value (or lack of value)
    of the variable is not changed.

    Rationale:

    In parent languages to CL, such behavior was documented. The omission of
    clear documentation in CL is presumably an accident.

    Current Practice:

    Most implementations already do not initialize the variable. Some
    implementations, however, assume that the missing initial value defaults
    to NIL and assume that the variable is always to be initialized if
    unbound.

    Adoption Cost:

    Some implementations suffer a minor incompatible change.  The
    modification to systems is presumably trivial.

    Benefits:

    It's sometimes useful to have the ability to declare a variable without
    initializing it. More importantly, though, DEFVAR is used by lots of
    users in every implementation and it deserves uniform treatment.

    Conversion Cost:

    It's stylistically poor to be relying on the variable to be initialized
    without writing the initial value explicitly anyway. Except for very
    rare situations where a conditional action is taken based on a BOUNDP
    test of the variable, user programs are unlikely to be affected in
    practice.

    Very few user programs are likely to be affected. The incidence rate is
    probably sufficiently low that the issue of automatic tools for
    conversion is irrelevant.

    Aesthetics:

    No significant issues are obvious.

    Discussion:

    The cleanup committee generally supports this clarification.

    [Version 3 was distributed at the last X3J13 meeting. This version has
    changed only to bring it in line with the current proposal format.]

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