Re: Typing in Ada

[Niklas Holsti <niklas.holsti@tidorum.fi>]

Hyman Rosen wrote:
> Nick Roberts wrote:
>  > That is to say that in most Ada
> 
>> programs it is typical to have constructions such as:
>>    type Apples is range 0..100;
>>    type Oranges is range 0..200;
> 
> 
> Is it really? The very fact that you have named these types as
> you did suggests that the appplicability of ranged types is quite
> limited. In a real program that does something useful, what kinds
> of things do you want to count up to 100 but not to 101? To 200
> but not to 201? Can you give examples from some of your real code

I'm not the original poster, but here are some examples from real code. 
I am implementing the mass memory subsystem for a satellite. The mass 
memory is divided into blocks, pages and octets. There are absolute 
address in the range 0 .. (total number of octets - 1), block numbers in 
the range 0 .. (total number of blocks - 1), page numbers likewise, 
octet offsets within a page, etc, etc. Using strong integer typing I can 
make the compiler check that I don't, by mistake, try to add a page 
number to an octet offset or do other equally senseless things.

To return to Nick Robert's example, the numbers 100 and 200 should 
either come from the requirements specification ("The program shall be 
able to count  up to 100 apples and 200 oranges...") or should appear in 
the software user manual ("The program can count up to 100 apples and 
200 oranges"). So then it is clear that the program satisfies its 
requirements (or it won't compile) and satisfies its user manual, 
whatever range Standard.Integer has on the current platform.

Niklas Holsti