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From: "cl1" <cwlambert76@yahoo.com>
Newsgroups: comp.lang.ada
Subject: issue with implementing a visitor design pattern
Date: Thu, 15 Jan 2004 13:22:32 -0600
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Date: 2004-01-15T13:22:32-06:00
List-Id: <comp.lang.ada>

i'm trying to implement a visitor pattern for a parse tree(its a simple
calculator at this point). i have an abstract type:

type Visitor_Record is abstract tagged null record;

with primative operations Visit() for each node type like so:

Visit(V : in out Visitor_Record; Node : in Add_Op'Class) is abstract;
...
Visit(V : in out Visitor_Record; Node : in Assignment_Node'Class) is
abstract;

what i'm trying to do is implement a Preorder_Visitor_Record type that uses
the Visit() operation to
traverse the tree and calls an operation Visit_Operation() that is also a
primative operation on Preorder_Visitor_Record
like so:
with parse_tree_pkg; use parse_tree_pkg; -- has the parse tree nodes a
primative operation Accept_Visitor() for each node
                                                                 -- and the
abstract Visitor_Record with primative operations Visit() for each node
package Preorder_Visitor_Pkg is
    type Preorder_Visitor_Record is new Visitor_Record with null record;

    -- this is overidiing the abstract version in parse_tree_pkg;
    procedure Visit ( V : in out Preorder_Visitor_Record; Node : in
Add_Op'Class);
    .....

    procedure Visit_Operation (V : in preorder_Visitor_Record; node : in
add_op'class);
end preorder_visitor_Pkg;

package body Preorder_Visitor_Pkg is

procedure Visit_Operation ( V : in out Preorder_Visitor_Record; Node : in
Add_Op'Class)
is begin
    raise Not_Implemented; -- better handling in actual code with an error
message using ada.exceptions
end Visit_Operation
procedure Visit( V : in out Preorder_Visitor_Record; Node : in Add_Op'Class)
is begin
    Visit_Operation(V, Node);
end Visit;


the actual code errors saying ambiguos call of Visit_Operation() from within
the Visit() procedure. with possible interpretations at the definition of
Preorder_Visitor_Record and at the specification for Visit_Operation.

the code works when you take Visit_Operation out of the mix. the package
compiles and works with a test driver. but when i try to add in this
Visit_Operation it doesn't work. I don't know if there are some scoping
issues or if there is something else i need to know about primative
operations to make this work. any guidance would be appreciated.

Thanks,
Charles Lambert