Ada: package does not allow a body

Question

Using the GNAT compiler, when I try to compile or check semantic on the following files, I get some variation of the error package "Foo" does not allow a body. I'm new to the language and can't find an answer to this seemingly basic problem anywhere on the Internet. Please advise.

foo.ads

package Foo is
   type Shape_Enum is (Circle, Triangle, Rectangle);
end Foo;

foo.adb

package body Foo is
   procedure Foo is
      MyShape : Shape_Enum;
   begin
      MyShape := Rectangle;
   end Foo;   
end Foo;

Once you get it working, I'd suggest renaming it to "Adarocks". Compilers are sensitive, and you'll want to try to get back on its good side. — T.E.D.
I've taken the liberty of editing the question to change the package name. — Keith Thompson
Whatever floats your boat. Thanks again for the answer. Accepted and upvoted. — weberc2

Keith Thompson Keith Thompson · Accepted Answer · 2012-09-27T22:16:17

A package is only allowed to have a body if the specification includes something that requires a body. (This avoid problems where an optional body might accidentally be left out of a build.)

You have a procedure in the body of the package (Foo.Foo), but there's no way to call it.

If you add a declaration:

procedure Foo;

to the specification, that should (a) fix the error, and (b) permit the procedure to be called by clients of the package. Or you can use pragma Elaborate_Body; to require it to have a body if you don't want the procedure to be visible to clients.

Incidentally, there's nothing special about a procedure with the same name as the package that contains it (unlike in C++, where such a function is a constructor for the containing class). It would probably be clearer to use a different name.

See section 7.2 of the Ada Reference Manual (I'm using a recent draft of the 2012 standard):

A package_body shall be the completion of a previous package_declaration or generic_package_declaration. A library package_declaration or library generic_package_declaration shall not have a body unless it requires a body; pragma Elaborate_Body can be used to require a library_unit_declaration to have a body (see 10.2.1) if it would not otherwise require one.

Ada: package does not allow a body

3 Answers