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                                       'XEROX
                             PALO ALTO RESEARCH CENTER
                              Computer Sciences Laboratory
                            INFORMATION PRODUCTS GROUP
                               System Development Division
                                      July 7, 1977

                               DRAFT - DRAFT - DRAFT



To:         A Sampling of Mesa Users
                                                           XEROI SDD ARCHIVES         d
                                                                               stoo
From:       Ed Satterthwaite, John Wick               1 have ' rea.d and under
Subject:    Variant Record Changes                    pe.ges_         ...-To   --
                                                                         Date_
                                                    'Reviewer
                                                                      _Ref 7 "'~b
Keywords:   Mesa, Variant Records                                      - ...... 7'-', D - cQ63
Filed On:    [b: BOOLEAN],             0:15 .. 15    1:0 .. 15
              blue => [c: CARDINAL],           1:0.. 15      1:0.. 15
              ENDCASE]


Note that, under the old scheme, bit fifteen of word zero of a blue Foo is unused
(garbage), hence blue Foos cannot be compared. On the other hand, in the new scheme,
both variants would occupy two words.

Note also that, under the new scheme, the tag field would not be widened if there were
some field in each variant that could be used to fill the remaining space. Thus if the
blue variant of Foo included some 1-bit field, a red Foo would occupy one word again
(at least if the record were not mutable; see below). If, as in this case, the variants were
of differing lengths, the compiler would still require discrimination before comparison.

Mutable Variant Records

There is a reasonably well known bug in the language definition that allows the type of
a variant record to be changed "on the fly", often wit.h disasterous consequences.
Consider the following code (assume the definition of Foo above):

       foo:   POINTER TO POINTER TO     Foo;
       baz:   POINTER TO Foo;

       WITH record: foot t      SELECT FROM
         red =>
              BEGIN
              foott +- Foo[3, blue[5]];
              IFrecord.b THEN 

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