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From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.lang.ada
Subject: Re: Constrained Boolean
Message-ID: <1039@aw.sei.cmu.edu>
Date: Mon, 20-Apr-87 09:01:34 EST
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Posted: Mon Apr 20 09:01:34 1987
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In article <3283@burdvax.PRC.Unisys.COM> eric@burdvax.PRC.Unisys.COM (Eric Marshall) writes:
>last night I wrote:
>> 
>>   The elaboration of 'a' below will not raise CONSTRAINT_ERROR.
>> 
>>   subtype bool is boolean range true..true;
>>   a : bool := true xor false;
>
>	This is a poor example, although my point is correct, XOR
>will not always raise CONSTRAINT_ERROR. Below is a more appropriate
>example:
>
>		subtype bool is boolean range false..false;
>		a, b : bool := false;
>		c : bool := a xor b;

You're right, Eric, and I made a rather stupid error.  The original
discussion was about a Boolean subtype constrained to one value.
Indeed, XOR works iff that one value is FALSE, since FALSE xor FALSE
yields FALSE.

In the case of XOR between arrays, the code then becomes something
like

	if BOOLEAN_SUBTYPE'FIRST = TRUE then
		raise CONSTRAINT_ERROR;
	else
		null;	-- never any need actually to DO the assignment!
	end if;

(I hope)