Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!cornell!uw-beaver!mit-eddie!husc6!ncar!ames!pacbell!vixie!paul
From: paul@vixie.UUCP (Paul Vixie Esq)
Newsgroups: comp.lang.modula2
Subject: Re: union types
Message-ID: <850@vixie.UUCP>
Date: 27 Mar 88 20:51:01 GMT
References: <8803221947.AA00248@nrl-iws6.ARPA> <4118@cup.portal.com>
Reply-To: paul@vixie.UUCP (Paul Vixie Esq)
Organization: Vixie Enterprises, San Francisco
Lines: 51

In article <4118@cup.portal.com> R_Tim_Coslet@cup.portal.com writes:
##Can anyone construct a suitable translation of:
##
##    struct obj_name {
##       int obj_type;
##       union {
##          int id;
##          char *path;
##       } obj_id;
##    };
#
#The direct equivalent of the above C in Modula-2 is....
#
#	ObjName : RECORD
#		    ObjType : INTEGER;
#		    CASE : BOOLEAN OF
#		      TRUE : id : INTEGER |
#		      FALSE : path : POINTER TO CHAR |
#		    END
#		  END
#
#
#This should creat the same data structure (and actually has one less
#identifier: obj_id).

Sometimes you *want* that intervening obj_id.  In C, it's harder (though
possible) to make a variant record where this intervening member needn't
be named in references to the variant fields; in M2, you can do it thus:

TYPE	ObjName = RECORD					(* note 1 *)
		ObjType: INTEGER;
		ObjId: RECORD
			CASE BOOLEAN OF				(* note 2 *)
				TRUE:   id: INTEGER|
				FALSE:  path: POINTER TO CHAR;	(* note 3 *)
			END
		END
	END;

Note 1: we are creating a type in the C example, not a variable.
Note 2: No ':' before the type as far as I know; [brackets] may be needed
	(I don't recall), and the type could be enumerated if more than
	two variants are needed -- BOOLEAN is convenient but not mandatory.
Note 3: POINTER TO CHAR is one way to represent strings, but sometimes arrays
	are used.  Sure would be great if open arrays were allowed in places
	other than a formal argument on a procedure...
-- 
Paul A Vixie Esq
paul%vixie@uunet.uu.net
{uunet,ptsfa,hoptoad}!vixie!paul
San Francisco, (415) 647-7023