Re: Static typing (Was Re: Better support for garbage collection)

[dmitry@elros.cbb-automation.de (Dmitry Kazakov)]

On Fri, 30 Mar 2001 12:44:57 -0800, "Robert C. Leif, Ph.D."
<rleif@rleif.com> wrote:

>From: Bob Leif
>To: Dmitry Kazakov et al.
>Are you proposing that Universal_Integer, Universal_Float,
>Universal_Character, etc. become visible types that can be explicitly used
>in type conversions including where an object of a type like integer can be
>converted to being of type Universal_Integer. Would it include subprograms
>like:
>function "+"   (Left : Integer'Base; Right : Universal_Integer) return
>Integer'Base;
>
>This would be very useful, since it would facilitate the use of private
>numeric types. I have yet to be able to cleanly use a private type
>Data_16_Type and directly add a literal to it.
>I would like:
>function To_Data_16_Type(The_Universal_Integer: Universal_Integer) return
>Data_16_Type;
>
>In essence these Universal_Types could now be used to explicitly specify the
>classes of integers, floats, and characters. I can see the danger of
>permitting programming with weakened types. However, these types could be
>included in a separate package Standard.Universal and be subject to the
>Pragma Restrictions or equivalent.

Yes and no. In my opinion instances of universal types may not exist
at run time. A universal_integer has an arbitrary length, its actual
representation is upon the compiler. So one may not have an object of
this type. In the end an instance of a universal type should be always
converted to some "legal" type. Yet with user-defined conversions one
will have an ability to explicitly add literals to a private type:

function <syntax-sugar> (Value : Integer) return Data_16_Type;

Then the following conversion path is applied:

123 (of universal_integer)->123 (of Integer)-> 123 (of Data_16_Type)

Further this issue is obviuosly relevant to another thread (where the
compile time routines are discussed). If such beasts were allowed then
one might have exactly what you mentioned:

function <syntax-sugar> (Value : Universal_Integer)
   return Data_16_Type;
??pragma <ensure-that-this-is-a-compile-time-function>;??

As for danger, it is immense, yet in modern times (take a look at
comp.object) strong-static-manifested typing is considered as an
evil?! (:-()

Regards,
Dmitry Kazakov