Re: Subtypes boundaries

[Adam Beneschan <adam@irvine.com>]

On Jun 14, 5:01 pm, "Peter C. Chapin" <pcc482...@gmail.com> wrote:
> Yannick Duchêne (Hibou57) wrote:
> > What disturbed me, is that a literal here, match an ASTERIX while it is  
> > statically out-of range ?
>
> I think that's accepted because the literal is a Universal Integer.

No, that's not the reason.  In fact, with one compiler, the above code
gives me a warning, but if I change the out-of-range upper bound by
sticking another zero on it:

subtype IDEFIX is ASTERIX range 1..200_000;

then it gives me an error and rejects the program---as I expected.

Here's the reason: The only error that has to be caught at compile
time is when the value is out of the *base* *range* of the expected
type (see 4.9(35)).  The base range for ASTERIX is not -5000 .. 10000;
it's some other range selected by the compiler, and I think in this
case this compiler probably selects
-32768 .. 32767.

I can understand why this all might be confusing to a newcomer.  I'm
not exactly sure why the language rules were done this way.  But I do
know that good practice often means writing programs in such a way
that you should be able to change a program's behavior by changing a
constant without having to go through the whole rest of the code
figuring out what else might have to be changed (sorry, I forgot the
term for that), and that means that sometimes a program will have IF
statements where the condition will always be false (or true), and you
don't want to have language rules that force a program to be rejected
if it contains a statement that is statically known to fail inside a
block of code that will never be executed.  Maybe that was the
motivation.

                               -- Adam