Re: Top 10 Language Constructs (Ada)

[David Kristola <David95037@See-My.Sig>]

On Sat, 15 Jul 2000 16:36:15 -0700, Marc A. Criley wrote
(in message <3970F56F.F3A70FAD@icdc.com>):

> The critical foundation of the Ada programming language is its "type model". 
> (In claiming this I want to credit Doug Bryan for triggering this insight 
> for me at an Ada conference seminar about a dozen years ago.)

I'd like to second David Botton's "Dude!".

Besides being strongly typed, Ada has a very robust type model.  Just
the ability to create subtypes with limited ranges seems to escape many
popular languages.  Add to that the long list of types, and you have a
very powerful means of expressing information.

The other day, i came across a message field in an interface document
that i was coding against.  The field is 16 bits, with the high order
bit being a sign bit, the next highest order bit has the value 2^1,
the third highest bit is 2^0, and so on down to the low order bit,
which is 2^(-13).  The note said that the field is 2's complement.
No problem....

   type Plasma_Volts_Type is delta 0.00012207 range -4.0 .. 3.99987793;
   for Plasma_Volts_Type'Size use 16;

I love fixed point types!  I happen to know that the software on the
other side of that interface is in C.  I wonder how they deal with
that field.

Another important feature of Ada types that people seem to miss:
a type does not have to have a size.  Take boolean for example.  Is
it 8 bits or 1?  That all depends on where you put it.  I could pack
8 boolean flags into a byte, or i could take up that whole byte with
just 1 boolean.  My needs dictate how i use the type.

A word of caution.  Types are the foundation of many programs.
Poorly laid out types can cause all kinds of problems.  I've seen
a case where a bad type choice at a fundamental level caused type
converts to be needed in hundreds of places throughout the code.


-- 
--djk, keeper of arcane lore & trivial fluff
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