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B.2 The Package Interfaces

1
Package Interfaces is the parent of several library packages that declare types and other entities useful for interfacing to foreign languages. It also contains some implementation-defined types that are useful across more than one language (in particular for interfacing to assembly language). 
1.a
Implementation defined: The contents of the visible part of package Interfaces and its language-defined descendants.

Static Semantics

2
The library package Interfaces has the following skeletal declaration: 
3

package Interfaces is
   pragma Pure(Interfaces);
4
   type Integer_n is range -2**(n-1) .. 2**(n-1) - 1;  --2's complement
5
   type Unsigned_n is mod 2**n;
6
   function Shift_Left  (Value : Unsigned_n; Amount : Natural)
      return Unsigned_n;
   function Shift_Right (Value : Unsigned_n; Amount : Natural)
      return Unsigned_n;
   function Shift_Right_Arithmetic (Value : Unsigned_n; Amount : Natural)
      return Unsigned_n;
   function Rotate_Left  (Value : Unsigned_n; Amount : Natural)
      return Unsigned_n;
   function Rotate_Right (Value : Unsigned_n; Amount : Natural)
      return Unsigned_n;
   ...
end Interfaces;

Implementation Requirements

7
An implementation shall provide the following declarations in the visible part of package Interfaces:
8
8.a
Ramification: For example, for a typical 32-bit machine the corresponding types might be Integer_8, Unsigned_8, Integer_16, Unsigned_16, Integer_32, and Unsigned_32.
8.b
The wording above implies, for example, that Integer_16'Size = Unsigned_16'Size = 16. Unchecked conversions between same-Sized types will work as expected. 
9
9.a
Reason: We considered making shifting and rotating be primitive operations of all modular types. However, it is a design principle of Ada that all predefined operations should be operators (not functions named by identifiers). (Note that an early version of Ada had "abs" as an identifier, but it was changed to a reserved word operator before standardization of Ada 83.) This is important because the implicit declarations would hide non-overloadable declarations with the same name, whereas operators are always overloadable. Therefore, we would have had to make shift and rotate into reserved words, which would have been upward incompatible, or else invent new operator symbols, which seemed like too much mechanism. 
10
10.a
Implementation Note: The names for these floating point types are not specified. {IEEE floating point arithmetic} However, if IEEE arithmetic is supported, then the names should be IEEE_Float_32 and IEEE_Float_64 for single and double precision, respectively.
10.1/2
   {AI95-00204-01} Support for interfacing to any foreign language is optional. However, an implementation shall not provide any attribute, library unit, or pragma having the same name as an attribute, library unit, or pragma (respectively) specified in the following clauses of this Annex unless the provided construct is either as specified in those clauses or is more limited in capability than that required by those clauses. A program that attempts to use an unsupported capability of this Annex shall either be identified by the implementation before run time or shall raise an exception at run time. 
10.b/2
Discussion: The intent is that the same rules apply for language interfacing as apply for Specialized Needs Annexes. See 1.1.3 for a discussion of the purpose of these rules. 

Implementation Permissions

11
An implementation may provide implementation-defined library units that are children of Interfaces, and may add declarations to the visible part of Interfaces in addition to the ones defined above.
11.a/2
Implementation defined: Implementation-defined children of package Interfaces. The contents of the visible part of package Interfaces.
11.1/2
   {AI95-00204-01} A child package of package Interfaces with the name of a convention may be provided independently of whether the convention is supported by the pragma Convention and vice versa. Such a child package should contain any declarations that would be useful for interfacing to the language (implementation) represented by the convention. Any declarations useful for interfacing to any language on the given hardware architecture should be provided directly in Interfaces.
11.b/2
Ramification: For example, package Interfaces.XYZ_Pascal might contain declarations of types that match the data types provided by the XYZ implementation of Pascal, so that it will be more convenient to pass parameters to a subprogram whose convention is XYZ_Pascal. 

Implementation Advice

12/2
 This paragraph was deleted.{AI95-00204-01} For each implementation-defined convention identifier, there should be a child package of package Interfaces with the corresponding name. This package should contain any declarations that would be useful for interfacing to the language (implementation) represented by the convention. Any declarations useful for interfacing to any language on the given hardware architecture should be provided directly in Interfaces.
12.a/2
This paragraph was deleted.Ramification: For example, package Interfaces.XYZ_Pascal might contain declarations of types that match the data types provided by the XYZ implementation of Pascal, so that it will be more convenient to pass parameters to a subprogram whose convention is XYZ_Pascal. 
13
An implementation supporting an interface to C, COBOL, or Fortran should provide the corresponding package or packages described in the following clauses. 
13.a.1/2
Implementation Advice: If an interface to C, COBOL, or Fortran is provided, the corresponding package or packages described in Annex B, “Interface to Other Languages” should also be provided.
13.a
Implementation Note: The intention is that an implementation might support several implementations of the foreign language: Interfaces.This_Fortran and Interfaces.That_Fortran might both exist. The “default” implementation, overridable by the user, should be declared as a renaming: 
13.b
package Interfaces.Fortran renames Interfaces.This_Fortran;

Wording Changes from Ada 95

13.c/2
{AI95-00204-01} Clarified that interfacing to foreign languages is optional and has the same restrictions as a Specialized Needs Annex.

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