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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Interfacing Ada with C
Date: Fri, 06 Aug 2010 21:26:18 +0100
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Date: 2010-08-06T21:26:18+01:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Fri, 06 Aug 2010 17:49:14 +0100, Simon Wright wrote:
>
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>> 
>>> On Fri, 06 Aug 2010 10:04:24 +0200, Jacob Sparre Andersen wrote:
>>>
>>>> Ada novice <posts@gmx.us> wrote:
>>>> 
>>>>> Can we change the subtype to Long_Long_Float? This will be of 18
>>>>> precision digits [...]
>>>> 
>>>> That's not guaranteed.  If you want 18 digits, you should declare the
>>>> type (My_Float?) as:
>>>> 
>>>>    type My_Float is digits 18;
>>>
>>> Further, practically you should never use subtype in the form
>>>
>>>    subtype My_Float is Long_Float;
>>>
>>> except for special cases like:
>>>
>>>    subtype Non_IEEE_Float is Long_Float range Long_Float'Range;
>>>
>>> or
>>>
>>>    subtype Non_Negative_Float is Long_Float range 0.0..Long_Float'Last;
>> 
>> Depends what you mean by "practically".
>> 
>> The thought process was, "I'm going to be writing a set of demo cases
>> which should work for arbitrary floating-point types, I'll call the type
>> My_Float. Shall I use type My_Float is new ... or subtype My_Float is
>> ...? Doesn't really matter, go for subtype."
>
> I am not sure what do you mean. If the code meant to be used with any type,
> then it should be generic (My_Float is digits <>). If the code is to work
> with some yet unknown type (to be determined later) then it should be
> My_Float is new Long_Float. That would protect me from mixing Long_Float
> and My_Float.

You're clearly right for serious code (in this case, for serious unit
tests which I may well make generic). But this wasn't a serious test, it
was an exploratory hack. Perhaps I shouldn't have let it out into the
wild without a bit more polishing.

I do feel that writing a demo like this one as a set of generics
instantiated for various floating types would probably have obscured the
point.