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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: How to check a Float for NaN
Date: Fri, 23 May 2014 16:39:54 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:20035
Date: 2014-05-23T16:39:54-05:00
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"Adam Beneschan" <adambeneschan@gmail.com> wrote in message 
news:b96fb365-9061-4170-8824-f68bab37f9dc@googlegroups.com...
...
>> Why doesn't it preclude NaN returned and accepted by arithmetic 
>> operations?
>> Either 3.5.7 lies or the operations do.
>
>I think it *does* preclude NaN, unless I don't understand what you mean.  I 
>guess you could
>say that the "operations lie", because if Machine_Overflows is False, an 
>operation on two
>values of a floating-point type may have an invalid result.

Right. I have no idea what's supposed to happen if Machine_Overflows is 
False. To the point that Janus/Ada simply doesn't use it; Machine_Overflows 
is True and we check after every group of operations to ensure that's true. 
So it's not possible to generate a NaN in Janus/Ada. I understand that the 
other semantics exists, but it's a mess by any standard and I'd need a lot 
more convincing ($$$$) to support something else. [That is, I see no 
sensible reason for NaNs or infinities -- they're just ways of deferring 
detection of bugs. I would have hoped that Ada's moved beyond that, just 
like it has for integers.]

                                 Randy.