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From: gamache@rapnet.sanders.lockheed.com
Newsgroups: comp.lang.ada
Subject: Re: Why don't large companies use Ada?
Date: 8 Dec 94 09:27:43 -500
Organization: Lockheed Sanders, Inc.
Message-ID: <1994Dec8.092743.1@rapnet.sanders.lockheed.com>
References: <3bj07h$qus@felix.seas.gwu.edu> <3blc1s$8gr@network.ucsd.edu>
 <3btf4n$jma@felix.seas.gwu.edu>
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Date: 1994-12-08T09:27:43-05:00
List-Id: <comp.lang.ada>

In article <3btf4n$jma@felix.seas.gwu.edu>, mfeldman@seas.gwu.edu (Michael Feldman) writes:
> In article <3blc1s$8gr@network.ucsd.edu>,
> Matt Kennel <mbk@inls1.ucsd.edu> wrote:
> 
[snip]
>>
>>What system of lengths/time/masses? 
> 
> Could be adapted to MKS, CGS, or both, with conversion constants.
> It's really intended to show the technique, not to be exhaustive.
>>

[more snip]

> BTW - the variant-record solution is not at all original with me;
> it's been sketched out several times in the literature. I take credit
> only for boiling it down and making a pretty complete package out of
> it, so that it shows how to handle dimensions and also how Ada does
> safe variant records.
> 
> Your "nondimensional" variables are fine, and in Ada would be done,
> presumably, by just creating a bunch of nicknames for Float (say):
> 
>   SUBTYPE Length IS Float;
>   SUBTYPE Mass IS Float;
> 
> etc. I have no problem with this approach. Recall that the thread
> started with a guy who said Ada's type system gets in the way of
> declaring _new_ or _derived_ types to handle units, e.g.
> 
>   TYPE Length IS DIGITS 6;
>   TYPE Mass iS DIGITS 6;
> 
This seemingly innocuous example hits one of my hot buttons. 

[climb soapbox]

So much damage can be done with a bad example it never ceases to amaze me.  
(ok, so this is why I'd rather mostly read CLA than post...!)  I've always
thought that K&R did so much harm to this profession with the examples it
contained.  I don't know about anyone else, but I still see people who think
it is acceptable to name file pointers "fp".

How does this relate to the above?  Well, I always view user defined types as
not only a way to provide contraints, but also as a means to provide 
*information* to some future reader of the code (which could possibly even
be the author his/her self).  This example, which *some* people will think
is an okay way to code fails to capitalize on this opportunity to convey
some information.  For instance, object definitions might look like:

	Obj1_Mass	: Mass := 0.001;
	Obj2_Mass	: Mass := 3.5;
the type definition fails to provide readers with any useful information.  This
is particularly noticable when people use _type on type definitions.  Then you
frequently see:

	Mass	: Mass_Type;

Few such simple issues get to me as much.  I even wish compilers would disallow
this construct altogther.  What would a future reader like to
know?  Is mass in kilograms, grams, milligrams?  Is length feet, meters, miles?
Code could then look like:

	Obj1_Mass	: Grams := 0.001;

which conveys much different information than:

	Obj2_Mass	: Kilograms := 0.001;

IMO, the distinction between this and the above is important.  Unfortunately, 
too much code that I see uses types such as length, mass etc.
[ok, climb down from soapbox...]

-------------------------------------------------------------------------------
Joe Gamache
Lockheed Sanders Inc.
with all_disclaimers_standard_or_otherwise;

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