Hi Folks,
Problem:
There are N nodes. Create an XML instance so that the nodes
form a ring (cyclic) topology.
Rick Jelliffe
solved this problem very elegantly [1].
Here’s Rick’s
solution: Arrange the nodes in document order:
<Nodes>
<Node
id="N1">...</Node>
<Node
id="N2">...</Node>
...
<Node
id="Nn">...</Node>
</Nodes>
“How does that
form a ring topology?” you ask. It doesn’t. Not yet. You need
to define an operator on the nodes and give the operator the
desired behavior (semantics). Allow me to explain.
To traverse from
one <Node> element to the next, you
must perform an operation. If you are moving from
<Node> to <Node> using your eyes, then the
operator is “physically move your eyes downward (or upward).”
If you are moving from <Node> to <Node> using
XPath, then the operator is following-sibling (or
preceding-sibling). Regardless of the technology, some
operator must be used to move from one <Node> element to
the next. Let’s generically denote that operator by
succ. To achieve our ring topology goal, then, we need
to imbue the succ operator with this semantic:
The succ of a <Node> is the next node in
document order
except when <Node> is the last node, then
the succ is the
first node.
Now the nodes
take on a ring topology, by way of the
succ operator.
Awesome! Thanks
Rick!
Lesson Learned:
XML has no semantics. When we define operators on the XML and
imbue specific behaviors on the operators, only then we may
informally treat the XML as having semantics.
Comments?
/Roger
[1] http://xml-dev.markmail.org/message/b3n774tmzqfe5k4d?q=Notable+declarative+expressions%3F+date:201612
