Path: utzoo!attcan!uunet!mcvax!hp4nl!htsa!fransvo
From: fransvo@htsa.uucp (Frans van Otten)
Newsgroups: comp.lang.modula2
Subject: What is the use of tag fields and case labels in records ?
Message-ID: <724@htsa.uucp>
Date: 24 Jan 89 19:09:39 GMT
Reply-To: fransvo@htsa.UUCP (Frans van Otten)
Organization: AHA Technical Institute  Amsterdam  The Netherlands
Lines: 31

In Modula-2, an 'union' is defined like:

    TYPE Example = RECORD
                     CASE : CARDINAL
                       OF
                          1 .. 10 : a : INTEGER;
                                    b : BOOLEAN  |

                         11 .. 15 : u : CHAR;
                                    v : ARRAY[1 .. 10] OF CHAR

                         ELSE       z : CHAR
                       END
                   END;

Can anyone explain to me what the tag type and the caselabels are
used for, apart from informing the reader what the intentions of
the programmer might have been ?

I know that a tag field can be included, but what is the real
difference between an ordinary field and a tag field ? The contents
of the tag field are variable and thus unknown at compile time, so
the compiler can't check if you're using the correct variant. At
run time this is not possible either because the computer can't tell
which variant field you are accessing.

-- 
                         Frans van Otten
                         Algemene Hogeschool Amsterdam
			 Technische en Maritieme Faculteit
                         fransvo@htsa.uucp