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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: A trouble with different instantiations of generic packages
Date: Wed, 29 Nov 2017 01:22:12 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:49229
Date: 2017-11-29T01:22:12+02:00
List-Id: <comp.lang.ada>

This problem appears when I try to "parse" (that is analyze like an abstract
syntax tree) an RDF directed graph. I think similar problem would appear with
a regular file parser.

I realize that I described the problem domain not very clearly, but I ask a
general question about generic packages, not about parsers specifically.

I have the following basic generic packages:

   -- Parser for subnodes (identified by Predicate) of Node
   generic
      type Data_Type (<>) is private;
   package Base_Predicate is

      type Base_Predicate_Parser is abstract tagged limited
         record
            Predicate: URI_Type;
         end record;

      not overriding function Parse (Context: Parser_Context_Type'Class;
                                     Parser: Base_Predicate_Parser;
                                     Model: Model_Type_Without_Finalize'Class;
                                     Node: Node_Type_Without_Finalize'Class)
                                     return Data_Type
                                     is abstract;

   end Base_Predicate;

   -- Parser for a node
   generic
      type Data_Type (<>) is private;
   package Base_Node is

      type Base_Node_Parser is abstract tagged limited
         null record;

      not overriding function Parse (Context: Parser_Context_Type'Class;
                                     Parser: Base_Node_Parser;
                                     Model: Model_Type_Without_Finalize'Class;
                                     Node: Node_Type_Without_Finalize'Class)
                                     return Data_Type
                                     is abstract;

   end Base_Node;

   -- Parser which ensures a node has exactly one subnode (for Predicate)
   generic
      type Child_Type (<>) is private;
   package One_Predicate is
      package Node_Parser is new Base_Node(Child_Type);
      package Predicate_Parser is new Base_Predicate(Child_Type);
      type One_Predicate_Parser is new Predicate_Parser.Base_Predicate_Parser with
         record
            Child_Parser: access Node_Parser.Base_Node_Parser'Class;
         end record;
      overriding function Parse (Context: Parser_Context_Type'Class;
                                 Parser: One_Predicate_Parser;
                                 Model: Model_Type_Without_Finalize'Class;
                                 Node: Node_Type_Without_Finalize'Class)
                                 return Child_Type;
   end One_Predicate;

Then I derive a specific parser like this:

   package Float_Node is new Base_Node(Long_Float);

   type Float_Literal_Parser is new Float_Node.Base_Node_Parser with null record;

   overriding function Parse (Context: Parser_Context_Type'Class;
                              Parser: Float_Literal_Parser;
                              Model: Model_Type_Without_Finalize'Class;
                              Node: Node_Type_Without_Finalize'Class)
                              return Long_Float;

But when I try to create an object of type One_Predicate_Parser with
Float_Literal_Parser child access, it fails because
Float_Node.Base_Node_Parser is not the same (a different instantiation) as
One_Predicate.Node_Parser.Base_Node_Parser.

What to do?

Maybe I should use in One_Predicate a package formal generic parameter like:

   package Node_Parser is new Base_Node(Child_Type);

?

Are there better ways?

-- 
Victor Porton - http://portonvictor.org