Re: package dependence question

[Ray Blaak <blaak@infomatch.com>]

Jeff Carter <jrcarter@acm.org> writes:
> Carl Banks wrote:
> > So let's say that a particular tree has a different type of node at
> > each level.  For example, type A is the root of the tree.  Its
> > branches are of type B.  B's branches are of type C, and so on.
> 
> Then this isn't really a tree; it's a hierarchy. Trees have the same
> type everywhere a value is stored. Hierarchies are shaped like trees,
> but are used differently.

I disagree. A tree is simply a directed graph without cycles. There is no
requirement that all nodes must be of the same type. In particular, a
hierarchy is a tree.

However, in an OO implementation of a tree, all nodes will usually be of a
common type so that one can specify parent/child relationships in one
place. There can certainly be more specialized types for different kinds of
nodes. 

E.g.

 type TreeNode is tagged record
 end record;

 -- common stuff for all nodes, especially knowledge of the parent, child
 -- containers, etc.
 function ParentOf(n : TreeNode) : TreeNode;
 function ChildCount(n : TreeNode) : Natural;
 function ChildAt(n : TreeNode; index : Natural) : TreeNode;
 procedure InsertChild(child : in out TreeNode; intoParent : in out TreeNode);

 -- root node specialization
 type A is new TreeNode with ... end record;

 type B is new TreeNode with ... end record;

 -- leaf nodes?
 type C is new TreeNode with ... end record;

In this case the nodes are simultaneously of the same type (i.e. TreeNode) and
yet are different types (A, B, C, ...).

-- 
Cheers,                                        The Rhythm is around me,
                                               The Rhythm has control.
Ray Blaak                                      The Rhythm is inside me,
blaak@infomatch.com                            The Rhythm has my soul.