Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!pasteur!agate!ig!uwmcsd1!csd4.milw.wisc.edu!markh
From: markh@csd4.milw.wisc.edu (Mark William Hopkins)
Newsgroups: comp.lang.modula2
Subject: Re: Recursively defined data types.
Summary: Referencing components of recursive data types.
Message-ID: <4641@uwmcsd1.UUCP>
Date: 17 Feb 88 03:35:24 GMT
References: <4593@uwmcsd1.UUCP> <11100002@snail>
Sender: daemon@uwmcsd1.UUCP
Reply-To: markh@csd4.milw.wisc.edu (Mark William Hopkins)
Organization: University of Wisconsin-Milwaukee
Lines: 27

In article <11100002@snail> carroll@snail.CS.UIUC.EDU writes:
>
>	I still don't understand how you would reference a recursive
>data element that was far down in the structure. How would you parametrize
>the reference to, say, the 10th data field of a recursively defined type?
>Would you have to type in 10 "." operators, with the field names
>between them? And how would you handle variable depth references?

If Modula-2 had data type recursion, then roughly like this:
List would be defined by:
	
                  List = RECORD CASE Null:Boolean OF
				   TRUE:              |
				   FALSE: Data:Atom;
					  Next:List
                                END
                         END;

<ACCESING THE TENTH ELEMENT OF THE LIST L:List>

ReadHead is declared to be of type List:

		    ReadHead := L;
		    FOR I := 1 TO 9 DO
			 ReadHead := ReadHead.Next
                    END;
		    RETURN ReadHead.Data;