Re: Constructing a modular type based on the size of an arbitrary type

[Mark A Biggar <mark.a.biggar@lmco.com>]

Mike Bates wrote:
> 
> I am writing a generic package in which I need to be able to construct
> a modular type with the same size in bits as an arbitrary packed type
> passed in as a generic parameter.  (This is to be able to do unchecked
> conversions between the arbitrary type and modular types; so they need
> to be the same size.)  This is what I tried:
> 
> generic
>     type Arbitrary_Type is private;
> package Foo is
> 
>     type Modular_Type is mod Arbitrary_Type'Size ** 2;
> 
> end Foo;
> 
> The definition of Modular_Type fails because the value
> Arbitrary_Type'Size ** 2 is not static.  (This surprises me, because
> the value is determinable at compile time.)
> 
> I also tried the following, relying on the instantiator to pass in
> the value of Arbitrary_Type'Size ** 2:
> 
> generic
>     Arbitrary_Modulus : Natural;
> package Foo is
> 
>     type Modular_Type is mod Arbitrary_Type;
> 
> end Foo;
> 
> This also fails, again because the modulus expression is not
> static, even though it seems that it should determinable at compile
> time.  It fails even if I use a numeric literal as the actual
> parameter in the instantiation.
> 
> Is there any way to construct a modular type to match the size of
> an arbitrary type?  Or is this as impossible as trisecting the angle?

Unfortunately, I beleive the only way to handle this is to move the 
burden of defining the appropriate modular type out of the generic
and pass it in as a generic parameter along with the conversion
functions.

generic
	type Arbitrary_type is private;
	type Abritrary_mod is mod <>;
    with function To(X: Arbitrary_Type) return Arbitrary_Mod;
    with function From(X: Arbitrary_Mod) return Arbitrary_Type;
package Foo is
	...
end Foo;

If you want to make sure it is set up right you can have package Foo
check
at run time in the body.

package body Foo is
	...
begin
	if Arbitrary_Type'SIZE /= Arbitrary_Mod'SIZE then
		raise Constraint_Error;
	end if;
end Foo;

Yes, I know that this moves a lot of the provate stuff of the package
outside, but I don't beleive there is any other way.

--
Mark Biggar
mark.a.biggar@lmco.com