CLHS: Issue TYPE-OF-AND-PREDEFINED-CLASSES Writeup

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## Issue TYPE-OF-AND-PREDEFINED-CLASSES Writeup

    Issue:          TYPE-OF-AND-PREDEFINED-CLASSES
    Forum:          Cleanup
    References:     Draft 8/29/89, sections 2.2 and 2.3
                    Draft 8/29/89, TYPE-OF
                    Issue TYPE-OF-UNDERCONSTRAINED
                    Issue DATA-TYPES-HIERARCHY-UNDERSPECIFIED
                    Issue FUNCTION-TYPE
                    Issue FLOATING-POINT-CONDITION-NAMES
                    Issue READER-ERROR
                    Condition System, Version 18
    Category:       Change
    Edit History:   Version 1, 1/2/90 by Kim A. Barrett
                    Version 2, 2/16/90 by Kim A. Barrett (from Moon's comments)

    Problem Description:

     In the 8/29/89 Draft there are three different lists of built-in type
     specifiers, all of which are slightly different.  Since some of the
     differences seem to be simply oversights, it would probably be helpful if they
     could be merged together.

     The locations of these lists in the Draft are (1) Section 2.2, in the
     description of the type disjointness requirements in the subsection entitled
     "Type Relationships", (2) Section 2.3, in Figure 2-13 "Classes that correspond
     to pre-defined type specifiers", and (3) in the description of the function
     TYPE-OF.

     The following table indicates which type names do not appear in all three
     places.  The groupings were made to facilitate the discussion below.

                            (1)            (2)             (3)
      Group 1
      CONDITION             x                               x
      LIST                  x               x
      LOGICAL-PATHNAME
      REAL                  x               x
      RESTART               x                               x
      Group 2
      SHORT-FLOAT           special                         x
      SINGLE-FLOAT          special                         x
      DOUBLE-FLOAT          special                         x
      LONG-FLOAT            special                         x
      Group 3
      BROADCAST-STREAM      x
      CONCATENATED-STREAM   x
      ECHO-STREAM           x
      FILE-STREAM           x
      STRING-STREAM         x
      SYNONYM-STREAM        x
      TWO-WAY-STREAM        x
      Group 4
      SIMPLE-BIT-VECTOR     x
      SIMPLE-STRING         x
      SIMPLE-VECTOR         x
      Group 5
      SERIOUS-CONDITION     x
      SIMPLE-CONDITION      x
      WARNING               x
      Group 6
      FIXNUM                x
      BIGNUM                x

    Proposal (TYPE-OF-AND-PREDEFINED-CLASSES:UNIFY-AND-EXTEND)

     1. Make the following changes to the disjointness requirements specified in
     Section 2.2, subsection Type Relationships.  This supersedes issue
     DATA-TYPES-HIERARCHY-UNDERSPECIFIED and the corresponding sections of issue
     FUNCTION-TYPE.

     1a. Change the third bullet to

       "Among the types which correspond to predefined classes (listed in Figure
        {insert figure number here, currently 2-13 }) only the subtype
        relationships specified for those classes may exist.  Any type created by
        DEFSTRUCT, DEFCLASS, or DEFINE-CONDITION is disjoint from any of the
        listed types and from any other type created by these forms, except for
        type relationships explicitly established by specifying superclasses in
        DEFCLASS or DEFINE-CONDITION, or through the use of the :include option of
        DEFSTRUCT."

     1b. Strike bullets 4-6 and 21 (since they now follow from bullet 3).

     2. Make the following changes to the constraints on TYPE-OF.

     2a. Change the first constraint on TYPE-OF to be

       "For any object for which (TYPEP object type) is true when type is one of
        the types for which there is a corresponding predefined class listed in
        Figure { insert figure number here, currently 2-13 }, the result of
        TYPE-OF is a type specifier for which (SUBTYPEP (TYPE-OF object) type)
        returns T T, i.e., either the type or a subtype of it (a subtype that the
        implementation's SUBTYPEP can recognize)."

     2b. Change the fifth constraint on TYPE-OF to also include condition types
     defined with DEFINE-CONDITION and objects created with MAKE-CONDITION.

     3. Make the following additions to the set of predefined classes.

     3a. Add the following condition classes, with the specified class precedence
     lists:

      ARITHMETIC-ERROR (ARITHMENTIC-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      CELL-ERROR (CELL-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      CONDITION (CONDITION T)
      CONTROL-ERROR (CONTROL-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      DIVISION-BY-ZERO (DIVISION-BY-ZERO ARITHMENTIC-ERROR ERROR SERIOUS-CONDITION
                            CONDITION T)
      END-OF-FILE (END-OF-FILE STREAM-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      ERROR (ERROR SERIOUS-CONDITION CONDITION T)
      FILE-ERROR (FILE-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      FLOATING-POINT-INEXACT (FLOATING-POINT-INEXACT ARITHMENTIC-ERROR ERROR
                            SERIOUS-CONDITION CONDITION T)
      FLOATING-POINT-INVALID-OPERATION (FLOATING-POINT-INVALID-OPERATION
                            ARITHMENTIC-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      FLOATING-POINT-OVERFLOW (FLOATING-POINT-OVERFLOW ARITHMENTIC-ERROR ERROR
                            SERIOUS-CONDITION CONDITION T)
      FLOATING-POINT-UNDERFLOW (FLOATING-POINT-UNDERFLOW ARITHMENTIC-ERROR ERROR
                            SERIOUS-CONDITION CONDITION T)
      PACKAGE-ERROR (PACKAGE-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      PARSE-ERROR (PARSE-ERROR STREAM-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      PRINT-NOT-READABLE (PRINT-NOT-READABLE ERROR SERIOUS-CONDIITON CONDITION T)
      PROGRAM-ERROR (PROGRAM-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      SERIOUS-CONDITION (SERIOUS-CONDITION CONDITION T)
      SIMPLE-CONDITION (SIMPLE-CONDITION CONDITION T)
      SIMPLE-ERROR (SIMPLE-ERROR SIMPLE-CONDITION ERROR SERIOUS-CONDITION
                            CONDITION T)
      SIMPLE-TYPE-ERROR (SIMPLE-TYPE-ERROR SIMPLE-CONDITION TYPE-ERROR ERROR
                            SERIOUS-CONDITION CONDITION T)
      SIMPLE-WARNING (SIMPLE-WARNING SIMPLE-CONDITION WARNING CONDITION T)
      STORAGE-CONDITION (STORAGE-CONDITION SERIOUS-CONDITION CONDITION T)
      STREAM-ERROR (STREAM-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      STYLE-WARNING (STYLE-WARNING WARNING CONDITION T)
      TYPE-ERROR (TYPE-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      UNBOUND-SLOT (UNBOUND-SLOT CELL-ERROR ERROR SERIOUS-CONDITION CONDITION T)
      UNBOUND-VARIABLE (UNBOUND-VARIABLE CELL-ERROR ERROR SERIOUS-CONDITION
                            CONDITION T)
      UNDEFINED-FUNCTION (UNDEFINED-FUNCTION CELL-ERROR ERROR SERIOUS-CONDITION
                            CONDITION T)
      WARNING (WARNING CONDITION T)

     3b. Add the class RESTART, with a class precedence list of (RESTART T).

     3c. Add the class LOGICAL-PATHNAME, with a class precedence list of
     (LOGICAL-PATHNAME PATHNAME T).

     3d. Add the following classes with the specified class precedence lists:
      BROADCAST-STREAM              (BROADCAST-STREAM STREAM T)
      CONCATENATED-STREAM           (CONCATENATED-STREAM STREAM T)
      ECHO-STREAM                   (ECHO-STREAM STREAM T)
      FILE-STREAM                   (FILE-STREAM STREAM T)
      STRING-STREAM                 (STRING-STREAM STREAM T)
      SYNONYM-STREAM                (SYNONYM-STREAM STREAM T)
      TWO-WAY-STREAM                (TWO-WAY-STREAM STREAM T)
     Remove the 23rd bullet from Section 2.2, subsection Type Relationships,
     which lists the above types as being disjoint subtypes of STREAM.

     3e. Add the following classes from the CLOS specification, with the specified
     class precedence lists:
      BUILT-IN-CLASS                (BUILT-IN-CLASS CLASS STANDARD-OBJECT T)
      CLASS                         (CLASS STANDARD-OBJECT T)
      GENERIC-FUNCTION              (GENERIC-FUNCTION FUNCTION T)
      METHOD                        (METHOD T)
      METHOD-COMBINATION            (METHOD-COMBINATION T)
      STANDARD-CLASS                (STANDARD-CLASS CLASS STANDARD-OBJECT T)
      STANDARD-GENERIC-FUNCTION     (STANDARD-GENERIC-FUNCTION GENERIC-FUNCTION
                                            FUNCTION T)
      STANDARD-METHOD               (STANDARD-METHOD METHOD STANDARD-OBJECT T)
      STANDARD-OBJECT               (STANDARD-OBJECT T)
      STRUCTURE-CLASS               (STRUCTURE-CLASS CLASS STANDARD-OBJECT T)
      STRUCTURE-OBJECT              (STRUCTURE-OBJECT T)

     4. Allow implementations to modify the class precedence lists of classes
     which are not specified to include either STANDARD-OBJECT or STRUCTURE-OBJECT
     to include those classes, placed before the class T but after all of the
     other specified classes.

    Proposal (TYPE-OF-AND-PREDEFINED-CLASSES:FLOAT-SUBCLASSES)

     Add the following to TYPE-OF-AND-PREDEFINED-CLASSES:UNIFY-AND-EXTEND:

     3f. Add the following subclasses of FLOAT:  SHORT-FLOAT, SINGLE-FLOAT,
     DOUBLE-FLOAT, LONG-FLOAT.  It is implementation-defined which among these
     classes are pairwise disjoint.  If there are fewer than four internal
     representations for floats, then some of these classes will not have proper
     names.  Instead, the rules for handling fewer than four internal
     representations (described in Section 2.2 subsection Data Type Definition) are
     used to determine the principal type name, for which there will be a class
     which has a proper name.  The other types which share the internal
     representation will all share the principal class.  The class precedence list
     for the properly named classes are

        (class-name FLOAT REAL NUMBER T)

     where 'class-name' is the name of the class.  Thus we get the following rules
     to handle a situation where there are fewer than four internal representations
     of floats.

        1. If there is only one internal representation for floats, then there is
        one class, named SINGLE-FLOAT.  (FIND-CLASS class-name) returns this class
        for all four of the subclasses of FLOAT.  The class precedence list for
        this class is (SINGLE-FLOAT FLOAT REAL NUMBER T).

        2. If there are two internal representations for floats, then they can be
        arranged in either of the following ways:

        * Two classes are provided, named SHORT-FLOAT and SINGLE-FLOAT.
          (FIND-CLASS class-name) returns the class SINGLE-FLOAT for any of the
          names SINGLE-FLOAT, DOUBLE-FLOAT, or LONG-FLOAT.  The class precedence
          lists of these classes are:

            SHORT-FLOAT     (SHORT-FLOAT FLOAT REAL NUMBER T)
            SINGLE-FLOAT    (SINGLE-FLOAT FLOAT REAL NUMBER T)

        * Two classes are provided, names SINGLE-FLOAT and DOUBLE-FLOAT.
          (FIND-CLASS class-name) returns the class SINGLE-FLOAT for either of the
          names SHORT-FLOAT or SINGLE-FLOAT.  (FIND-CLASS class-name) returns the
          class DOUBLE-FLOAT for either of the names DOUBLE-FLOAT or LONG-FLOAT.
          The class precedence lists for these classes are:

            SINGLE-FLOAT    (SINGLE-FLOAT FLOAT REAL NUMBER T)
            DOUBLE-FLOAT    (DOUBLE-FLOAT FLOAT REAL NUMBER T)

        3. If there are three internal representations for floats, then they can be
        arranged in either of the following ways:

        * Three classes are provided, named SHORT-FLOAT, SINGLE-FLOAT, and
          DOUBLE-FLOAT.  (FIND-CLASS class-name) returns the class DOUBLE-FLOAT for
          either of the names DOUBLE-FLOAT or LONG-FLOAT.  The class precedence
          lists for these classes are:

            SHORT-FLOAT     (SHORT-FLOAT FLOAT REAL NUMBER T)
            SINGLE-FLOAT    (SINGLE-FLOAT FLOAT REAL NUMBER T)
            DOUBLE-FLOAT    (DOUBLE-FLOAT FLOAT REAL NUMBER T)

        * Three classes are provided, named SINGLE-FLOAT, DOUBLE-FLOAT, and
          LONG-FLOAT.  (FIND-CLASS class-name) returns the class SINGLE-FLOAT for
          either of the names SHORT-FLOAT or SINGLE-FLOAT.  The class precedence
          lists for these classes are:

            SINGLE-FLOAT    (SINGLE-FLOAT FLOAT REAL NUMBER T)
            DOUBLE-FLOAT    (DOUBLE-FLOAT FLOAT REAL NUMBER T)
            LONG-FLOAT      (LONG-FLOAT FLOAT REAL NUMBER T)

    Proposal (TYPE-OF-AND-PREDEFINED-CLASSES:TYPE-OF-HANDLES-FLOATS)

     Add the following to TYPE-OF-AND-PREDEFINED-CLASSES:UNIFY-AND-EXTEND:

     2c. Add the following additional constraint on TYPE-OF.

      "For any object for which (TYPEP object type) is true when type is one of
       the types SHORT-FLOAT, SINGLE-FLOAT, DOUBLE-FLOAT, or LONG-FLOAT, the
       result of TYPE-OF is a type specifier for which (SUBTYPEP (TYPE-OF object)
       type) returns T T, i.e, either the type or a subtype of it (a subtype that
       the implementation's SUBTYPEP can recognize)."

    Rationale:

     Changing the list of disjoint types and the first constraint on TYPE-OF to
     reference the table of predefined classes merges three places with almost
     identical information, resulting in simplification and improved cohesion.
     Note that one effect of this is to remove the subtypes of FLOAT (SHORT-FLOAT,
     SINGLE-FLOAT, DOUBLE-FLOAT, and LONG-FLOAT) from the list of type specifiers
     mentioned in the first constraint on TYPE-OF (but see the subproposals
     FLOAT-SUBCLASSES and TYPE-OF-HANDLES-FLOATS), and to add the types LIST
     (which is not necessary, since CONS and NULL form an exhaustive partition of
     LIST) and REAL (which is probably missing due to oversight).

     Adding DEFINE-CONDITION to the third bullet brings it inline with the intent
     of issue CLOS-CONDITIONS.

     Adding DEFINE-CONDITION and MAKE-CONDITION to the fifth constraint brings it
     inline with the intent of issue CLOS-CONDITIONS.

     Adding classes for the predefined condition types matches up with the
     disjointness requirements specified by the Condition System, and corresponds
     to the intent of issue CLOS-CONDITIONS.  It could be argued that those two
     places already mandate this addition, but this proposal ensures that we are
     agreed on that.

     Adding RESTART as a predefined class matches up with the disjointness
     requirements specified by the Condition System.  It could be argued that
     the Condition System already mandates the addition of this class, but this
     proposal ensures that we are agreed on that.

     Adding LOGICAL-PATHNAME as a predefined class seems inline with the
     expressed intent for making it a type.  It is sufficiently new that it may
     simply have not been propagated to all the places that should mention it.

     Adding the stream subclasses seems reasonable, since we are already
     requiring them to be disjoint subtypes of STREAM.

     Adding the CLOS classes fixes an oversight.

     Allowing implementations to add STANDARD-OBJECT or STRUCTURE-OBJECT to the
     class precedence lists allows them to implement any of the predefined classes
     using the standard metaclasses STANDARD-CLASS and STRUCTURE-CLASS.

     The two subproposals, FLOAT-SUBCLASSES and TYPE-OF-HANDLES-FLOATS, are two
     different ways we could undo the removal of the subtypes of FLOAT from the
     list of types mentioned by the first constraint on TYPE-OF.

    Current Practice:

     Up to some "bugs" because they haven't caught up with X3J13 on some of
     these yet, IIM mostly implements this proposal.

     Other implementations were not surveyed.

    Cost to Implementors:

     Redirecting the disjointness requirements to reference the list of
     predefined classes and modifying the constraints on TYPE-OF primarily
     affects documentation, especially in the writing of the Standard.  The
     requirements on implementors are largely unchanged.

     The effect of adding the various classes are probably minimal.
     Implementations which already have the STREAM subtypes as distinguished
     types can probably easily support them as classes too, since they are
     probably implemented using DEFSTRUCT, CLOS, FLAVORS, or some other similar
     mechanism, and making TYPE-OF return the right type name in this case is
     probably trivial.  Implementations which don't have these stream types
     already have some work to do, and the necessary support for the types will
     probably produce the necessary support for the other stuff.  Similarly for
     CONDITION and its subtypes, LOGICAL-PATHNAME, RESTART, and the CLOS classes.

    Cost to Users:

     Minimal.  Many of the types mentioned here are new.  The changes to TYPE-OF
     would probably be an improvement for most users.

    Benifits:

     Fewer distributed tables which need to be maintained while writing the
     Standard.

    Discussion:

     An alternative to the unification would be to fix TYPE-OF by adding REAL to
     the list of built-in types referenced by the first constraint, to modify the
     fifth constraint to mention DEFINE-CONDITION, and to modify the third bullet
     of the type relationships to also mention DEFINE-CONDITION.

     A consequence of this proposal is that the following type names do not have
     associated classes.  Perhaps we should add a table to the standard which
     lists these as specifically not required to have associated classes (though
     they may be classes in some implementations).

     Subtypes of INTEGER            Subtypes of CHARACTER
       BIGNUM                         BASE-CHARACTER
       BIT                            EXTENDED-CHARACTER
       FIXNUM                         STANDARD-CHAR
       SIGNED-BYTE
       UNSIGNED-BYTE

     Subtypes of ARRAY              Subtypes of FUNCTION            Other
       BASE-STRING                    COMPILED-FUNCTION               ATOM
       SIMPLE-ARRAY                                                   NIL
       SIMPLE-BASE-STRING           Subtypes of SYMBOL
       SIMPLE-BIT-VECTOR              KEYWORD
       SIMPLE-STRING
       SIMPLE-VECTOR

     Subtypes of FLOAT (if FLOAT-SUBCLASSES is not adopted)
       SHORT-FLOAT
       SINGLE-FLOAT
       DOUBLE-FLOAT
       LONG-FLOAT

     Barrett isn't entirely happy with the class precedence lists specified for
     the CLOS classes.  He feels that they might be overly constrained in some
     cases.  For example, it should be possible to have an implementation make
     STRUCTURE-CLASS a trivial subclass of STANDARD-CLASS, define DEFSTRUCT
     (without the :type option) in terms of DEFCLASS, and make STRUCTURE-OBJECT be
     a subclass of STANDARD-OBJECT.

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