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From: do_while@owens.ridgecrest.ca.us (Do-While Jones)
Subject: Re: Dimensional Data Types
Message-ID: <D0CF11.BDB@ridgecrest.ca.us>
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References: <D07KAr.767@ridgecrest.ca.us> <3briug$9ab@news-2.csn.net>
Date: Mon, 5 Dec 1994 15:00:37 GMT
Date: 1994-12-05T15:00:37+00:00
List-Id: <comp.lang.ada>

In article <3briug$9ab@news-2.csn.net> perez@oldcolo.com (Carlos Perez) writes:
>Do-While Jones (do_while@owens.ridgecrest.ca.us) wrote:
>
>: Since the USE clause makes the operators visible, I just write things like
>: "VELOCITY := DISTANCE / TIME;" and Ada does all the dimensional checking
>: for me.  If I write "VELOCITY := DISTANCE / VOLTS;", I naturally get an
>: error message.  Of course I can respond to the error message by doing
>: something really stupid, like "VELOCITY := DISTANCE / Seconds(VOLTS);", but
>: I am not that dumb yet.
>
>It appears to me that you have designed an idiot-resistant system
>(there is no such thing as idiot-proof ;-) 
>

I regret including that paragraph.  It caused more confusion than 
enlightenment.  I was trying to say that it is idiot-resistant but not 
idiot-proof.  I just didn't say it very well.

>I thought that your VOLTS was a private type and so you can't 
type >cast VOLTS into SECONDS.  You may need to have some functions whose
>sole purpose is to convert dimensional types into scalar (non-dim) types
>like Volts_To_Float and Float_To_Volts so you can define new and improved
>cross-dimensional operators such as "*". 
>
>My point is that Ada won't permit a simple cast as you described, unless
>I missed something? 
>

Yes, you did miss something.  Volts and Seconds are private types.  They 
are DERIVED types that happened to be derived from a private type.  So, 
you can do a simple type cast.  For example, this program compiles 
without error:

with TIME_UNITS;
with ELECTRICAL_UNITS;
procedure Really_Stupid is
  S : TIME_UNITS.Seconds;
  V : ELECTRICAL_UNITS.Volts;
  use ELECTRICAL_UNITS; -- for explicit type conversion
begin
  V := Volts(S);
end Really_Stupid;

Which just goes to show that if you try really hard, Ada will let you 
shoot yourself in the foot.  But you have to try hard to do it, and it is 
pretty obvious when you do.

>: Do-While Jones
>
>When-Others Perez 


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