From: Victor Porton <porton@narod.ru>
Subject: A trouble with different instantiations of generic packages
Date: Wed, 29 Nov 2017 01:22:12 +0200
Date: 2017-11-29T01:22:12+02:00 [thread overview]
Message-ID: <ovkr32$47q$1@gioia.aioe.org> (raw)
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
next reply other threads:[~2017-11-28 23:22 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-28 23:22 Victor Porton [this message]
2017-11-29 1:13 ` A trouble with different instantiations of generic packages Victor Porton
2017-11-29 9:04 ` Dmitry A. Kazakov
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