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From: josef@ugun21.UUCP
Newsgroups: comp.lang.modula2
Subject: Re: What is the use of tag fields and c
Message-ID: <16900003@ugun21>
Date: 26 Jan 89 10:18:00 GMT
References: <724@htsa.uucp>
Lines: 52
Nf-ID: #R:htsa.uucp:-72400:ugun21:16900003:000:1686
Nf-From: ugun21.UUCP!josef    Jan 26 11:18:00 1989


fransvo@htsa.UUCP (Frans van Otten) writes
>"What is the use of tag fields and case labels"

Well, in Your example, they are of no other use than to identify
the different variants of this record.
But one could add an identifier to the CASE, e.g.

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

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

                         ELSE       z : CHAR
                       END
                   END;

Then You use "tag" as a field of the record AND a selector for the variants.
A "useful" example is:
	TYPE CoordType = (cartesian, cylinder, polar);
	TYPE Point = RECORD
		CASE CoordSystem: CoordType OF
		cartesian: x, y: INTEGER |
		cylinder: elevation INTEGER;
			radius: CARDINAL;
			angle: REAL |
		polar: radius: CARDINAL;
			azimuth, elevation: REAL
		END
	END;
Now You can store information about the kind of coordinate system You are
using as well as the coordinates applicable.

IMHO The usage of the "variant record" (as it is called in PASCAL)
WITHOUT the named tag field is more or less a mis-use.
How will You ever find out which variant to use when You haven't
stored any hints (If You have stored any, You can use them as tag fields!)

		Josef Moellers

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