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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: Rough proposal to make some generic types static
Date: Mon, 21 Jul 2014 23:54:45 +0300
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Date: 2014-07-21T23:54:45+03:00
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Victor Porton wrote:

> What in the RM signifies that Enum_Type (below) is not a static scalar
> type?

I've found: RM 4.9 26/3:

"A static scalar subtype is an unconstrained scalar subtype whose type is 
not a descendant of a formal type, or ..."

Why this rule? Can it be relaxed?

> We should work on an amendment for a future version of Ada to make
> Enum_Type static.
> 
> Is it difficult to implement this?
> 
> Rationale is that the below code is useful in practice.
> 
> gnatmake -q -c -gnatc -u
> -P/home/porton/Projects/redland-bindings/ada/test.gpr -XRUNTIME=full
> -XMODE=Install rdf-auxilary.adb rdf-auxilary.adb:7:25: non-static
> expression used for modular type bound rdf-auxilary.adb:7:38: size
> attribute is only static for static scalar type (RM 4.9(7,8))
> 
> -- rdf.ads
> package RDF is
> end RDF;
> 
> -- rdf-auxilary.ads
> with Interfaces.C;
> 
> package RDF.Auxilary is
> 
>    generic
>       type Enum_Type is (<>);
>    package Convert_Enum is
>       function To_C(Argument: Enum_Type) return Interfaces.C.int;
>       function From_C(Argument: Interfaces.C.int) return Enum_Type;
>    end;
> 
> end RDF.Auxilary;
> 
> -- rdf-auxilary.adb
> with Ada.Unchecked_Conversion;
> 
> package body RDF.Auxilary is
> 
>    package body Convert_Enum is
> 
>       type Small is mod 2**(Enum_Type'Size);
> 
>       function To_C_Internal is
>         new Ada.Unchecked_Conversion (Source => Enum_Type, Target =>
>         Small);
> 
>       function From_C_Internal is
>         new Ada.Unchecked_Conversion (Source => Small, Target =>
>         Enum_Type);
> 
>       function To_C(Argument: Enum_Type) return Interfaces.C.int is
>       begin
>          return Interfaces.C.int(To_C_Internal(Argument));
>       end;
> 
>       function From_C(Argument: Interfaces.C.int) return Enum_Type is
>       begin
>          return From_C_Internal(Small(Argument));
>       end;
> 
>    end;
> 
> end RDF.Auxilary;
> 
> 
-- 
Victor Porton - http://portonvictor.org