Re: Typing in Ada

["Robert I. Eachus" <rieachus@comcast.net>]

Peter C. Chapin wrote:

> Yes, Ada provides ranges as distinct types without automatic conversions 
> and that's a nice feature. In theory you could get some of the same 
> benifits in C by introducing an abstract type (as a structure) to hold a 
> range. It could be done much more nicely in C++ using templates, 
> overloaded operators, and other C++ features. The reality is, though, that 
> many C (or C++) programs don't bother. Perhaps they should.
> 
> It's not clear to me how all this relates to the concept of strong typing. 
> I guess it would be helpful to have a precise definition of that term. It 
> sounds like from what we've said here that strong typing more a matter of 
> programming style than a language feature... at least where C and Ada are 
> concerned. Is that a valid conclusion?

I guess the real difference between C (or C++) and Ada is that Ada has 
many features you get "for free" at compile time if you use strong 
typing.  The most visible of these in Ada is overloading and overload 
resolution.  To extend the Apples and Oranges example, if I define

type Apple is (McIntosh, Granny_Smith, Winesap, Golden_Delicious,...);
type Orange is (Valencia, Navel, Mandarin, Seville, Bergamot...);

then

Eat(McIntosh); -- will call the Eat function for Apples, and
Eat(Navel);
  -- will call the Eat function for Oranges, and peel it first. ;-)

The most important benefit of this is that if a procedure or functions 
has a multitude of parameters some possibly with defaults, when 
programming in Ada, you don't have to "validate" the call against the 
function template.  The compiler will do that for you, and detect most 
errors.

Of course, if you write "C := A - B;" when you meant "C := B - A;"  the 
compiler probably won't catch that.  However there are cases where it 
will.  For example, Ada.Calendar defines:

   function "-" (Left : Time;   Right : Duration) return Time;
   function "-" (Left : Time;   Right : Time) return Duration;

But no operation for subtracting Time from Duration.  So Ada would catch 
a call to the first "-" with the parameters in the wrong order, but 
could not detect such an inversion if the parameters were both of type 
Time.

-- 

                                           Robert I. Eachus

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