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## Issue DEFSTRUCT-REDEFINITION Writeup

    Status:	Passed (as amended) Jan 89 X3J13
    Issue:          DEFSTRUCT-REDEFINITION
    References:     DEFSTRUCT (CLtL pp 305-320)
    Related-issues: DEFSTRUCT-ACCESS-FUNCTIONS-INLINE
    Category:       CLARIFICATION
    Edit history:   Version 1 by Skona Brittain 07/26/88
    		Version 2 by Larry Masinter  7-Jan-89
    		Version 3 by Masinter 6-Feb-89 as per Jan 89 X3J13 amendment

    Problem Description:

    The case of a structure type being redefined is not discussed in CLtL. Is
    it legal to redefine a DEFSTRUCT? What happens to DEFSTRUCTS that :INCLUDE
    the one defined. What things might be "wired in" in compiled code that
    refered to the previous DEFSTRUCT?

    Proposal: (DEFSTRUCT-REDEFINITION:ERROR):

    The results of redefining a DEFSTRUCT structure are undefined.

    Rationale:

    DEFSTRUCT is intended as "the most efficient" structure class. DEFCLASS
    allows much more flexible structures to be defined. Thus, implementations
    should be free to "wire in" much of the behavior of a DEFSTRUCT into
    compiled code.

    The issue of redefinition should be addressed since there are always
    consequences that affect use of the structures.

    Current Practice:

    None of KCL, Lucid, & Symbolics detect a redefinition.

    Envos Medley goes to some effort to detect if a new structure is
    "compatible" with the old -- e.g., slots might change names, initial
    values, but, since the space allocated in an instance is determined by the
    :TYPE, an incompatible set of :TYPE forms would cause old instances to be
    marked "obsolete". (The TYPE-OF an old instance changes to **OBSOLETE**,
    for example.)

    Cost to Implementors:

    This proposal attempts to be consistent with current practice.

    Cost to Users:

    It is doubtful that any current programs actually define structures more
    than once. Thus, constraints on DEFSTRUCT redefinition primarily affect the
    debugging environment.

    Cost of Non-Adoption:

    Confusion.

    Benefits:

    Clarity.

    Aethetics:

    Something that is not well-defined and leads to erratic behavior should be
    explicitly considered an error.

    Discussion:

    Common implementation techniques may cause the following behavior if a
    DEFSTRUCT is redefined:

    If the new DEFSTRUCT is identical to the old DEFSTRUCT except for the
    initialization forms for slots,  previous structure objects probably can
    continue to be accessed with previously compiled slot accessors. DEFSTRUCT
    constructor, test functions are proclaimed INLINE, and if these have
    changed, previously compiled occurrences of them may behave unpredictably.

    If any change is made to the definiton of the slots (either in number,
    name, or :TYPE), attempting to execute a slot accessor of the old
    definition may behave unpredictably: if a slot name of the old definition
    also names a slot of the new definition, any "compiled" code might use the
    old definition instead.

    DEFSTRUCT constructor, test functions may also be proclaimed INLINE, and
    may behave unpredictably if previously compiled. In particular, a compiled
    occurance of a constructor might have the previously slot initial values
    "wired in".

    If the new DEFSTRUCT differs from the old in any aspect other than the
    initialization forms for slots, the results of attempting to access any old
    instance might result in unspecified behavior. For example, if the size of
    the structure became considerably shorter, an old accessor might "access
    off the end" of an instance of a new object; it might signal an error or
    have other unpredictable results.

    Masinter supports this proposal.  If users want more flexibility than
    DEFSTRUCT allows, they should use DEFCLASS.

    Some felt strongly that  just saying it's an error to redefine a structure
    but not requiring the error to be signalled will cause users to be confused
    by the differing seemingly erratic behavior and code.

    Programming environments are allowed, encouraged, etc. to allow such
    redefinition, perhaps with warning messages. It is beyond the scope of the
    language standard to define those interactions, except to note that they
    are not portable.

    Here's an example where reexecuting an EQUAL DEFSTRUCT might result in
    different behavior:

    (defvar *token-counter* 0)
    (defstruct token (cookie '("unique-string")) (counter (incf
    *token-counter*)))

    (defvar *first-token* (make-token))

    (eql (token-cookie *first-token*) (token-cookie (make-token))) => true

    (defstruct token (cookie '("unique-string")) (counter (incf
    *token-counter*)))

    (eql (token-cookie *first-token*) (token-cookie (make-token))) => false

    I.e., even though the second DEFSTRUCT is EQUAL to the first, the
    structures are not EQL.

    This is related to the compiler issue QUOTE-MAY-COPY, but is not the same
    issue, since that proposal isn't proposing that QUOTE might copy its value
    *every time* it is executed.

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