XPath Help

1
XPath Help

• Karl Lieberherr

2
Source of information / Plan

• http//www.w3.org/TR/2005/WD-xpath20-20050915/
• Complex definition
• We want to extract the static part of XPath
• Objective Use / and // (//) to build sequences
  and use operators union, intersection and except
  to combine them.

3
Mismatches

• XPath can work without schema.
• But we want to visualize XPath expression in
  schema.
• Works only for static XPath expressions
• XPath views meaning of expression as a sequence.
• Strategies views the meaning of an expression as
  a function that maps objects to object graph
  slices. Traversing slice sequence of nodes.

4
Mismatches

• But we can also view the meaning of a strategy
  and a class graph as another graph
  TraversalGraph

5
XPath data model

• In the data model, a value is always a sequence.
• A sequence is an ordered collection of zero or
  more items.
• An item is either an atomic value or a node.
• singleton, empty sequence. Sequences are never
  nested.

6
EBNF for XPath

• 26   PathExpr      ("/" RelativePathExpr?)
  ("//" RelativePathExpr) RelativePathExpr
• 27   RelativePathExpr      StepExpr (("/"
  "//") StepExpr)
• 28   StepExpr      AxisStep FilterExpr

7
Class dictionaries / EBNF

• exercise3.txt
• class dictionaries are both a simplification and
  extension of EBNF.

8
EBNF for XPath

• 30   ForwardStep      (ForwardAxis NodeTest)
  AbbrevForwardStep
• 31   ForwardAxis      lt"child" ""gt
  lt"descendant" ""gt lt"attribute" ""gt
  lt"self" ""gt lt"descendant-or-self" ""gt
  lt"following-sibling" ""gt lt"following" ""gt
  lt"namespace" ""gt

9
EBNF for XPath

• 3.3.3 Combining Node Sequences
• 14   UnionExpr      IntersectExceptExpr (
  ("union" "") IntersectExceptExpr )
• 15   IntersectExceptExpr      InstanceofExpr
  ( ("intersect" "except") InstanceofExpr )
• 16   InstanceofExpr   PathExpr

10
sequence operations

• XPath provides the following operators for
  combining sequences of nodes
• The union and operators are equivalent. They
  take two node sequences as operands and return a
  sequence containing all the nodes that occur in
  either of the operands.
• The intersect operator takes two node sequences
  as operands and returns a sequence containing all
  the nodes that occur in both operands.
• The except operator takes two node sequences as
  operands and returns a sequence containing all
  the nodes that occur in the first operand but not
  in the second operand.

11
sequence operations

• seq1 is bound to (A, B)
• seq2 is bound to (A, B)
• seq3 is bound to (B, C)
• Then
• seq1 union seq2 evaluates to the sequence (A,
  B).
• seq2 union seq3 evaluates to the sequence (A,
  B, C).
• seq1 intersect seq2 evaluates to the sequence
  (A, B).
• seq2 intersect seq3 evaluates to the sequence
  containing B only.
• seq1 except seq2 evaluates to the empty
  sequence.
• seq2 except seq3 evaluates to the sequence
  containing A only.

12
Declare context node type

• what is meaning of / B and // B?

13
Mixing strategies and XPath??

• what would the meaning be of
• s1 from A via B to C
• s2 A // D
• union (s1 s2)