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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: A trouble with different instantiations of generic packages
Date: Wed, 29 Nov 2017 03:13:24 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:49232
Date: 2017-11-29T03:13:24+02:00
List-Id: <comp.lang.ada>

Yes, to use formal generic packages seems a solution.

Victor Porton wrote:
> 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