Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!dalcs!silvert
From: silvert@dalcs.UUCP
Newsgroups: comp.sys.atari.st
Subject: Type coercion
Message-ID: <2499@dalcs.UUCP>
Date: Sat, 4-Apr-87 08:48:52 EST
Article-I.D.: dalcs.2499
Posted: Sat Apr  4 08:48:52 1987
Date-Received: Sun, 5-Apr-87 08:49:43 EST
Organization: Marine Ecology Lab.
Lines: 48
Keywords: C, Modula-2, long, int, conversion

Since I occasionally post source to the net, I thought it might be good
to try to reach a consensus on whether type coercion is desirable in
portable code, or whether, as the books say, it is a despicable and
dangerous practice which ranks with goto's.

I refer specifically to date and time functions, which sometimes use two
separate integer/CARDINAL variables (I refer to C/Modula-2 here), but
sometimes pack both into a long/LONGCARD variable.  Conversion of one to
the other involves either arithmetic or bitswitching, and can lead to
problems such as the recently reported type-extension bug in settime and
autodisk.

Type coercion in this case means putting both formulations in the same
location by using a union:

	union { long datetime;
		structure { int date;
			    int time;
			  } pair
		} trick;

(apologies if I haven't got the syntax quite right)
or a variant record in Modula-2

	TYPE trick = RECORD
			CASE : BOOLEAN OF
			TRUE : datetime : LONGCARD |
			FALSE: date, time : CARDINAL
			END
		     END;

Then once you have obtained the value of datetime you have date and time
ready to work with, and vice versa, and the conversions are automatic.

I have used this in my own datesetting program and find it convenient,
but wonder if it creates problems with other compliers and languages.
Pascal also has the variant record feature, but this use of type
coercion is illegal -- whether the compilers on the ST permit it I don't
know.  Clearly this usage assumes that structures are stored
consecutively, which I think is always the case.

Anyway, comments on this usage would be appreciated.  If I get a lot of
mail, I will post the conclusions to the net.
-- 
Bill Silvert, Modelling/Statistics Group
Bedford Institute of Oceanography, Dartmouth, NS, Canada
CDN or BITNET: silvert@cs.dal.cdn	-- UUCP: ..!{seismo|utai}!dalcs!silvert
ARPA: silvert%dalcs.uucp@seismo.CSS.GOV	-- CSNET: silvert%cs.dal.cdn@ubc.csnet