XSLT Xpath - PowerPoint PPT Presentation

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XSLT Xpath

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Variables shared between pattern and result allow movement of data ... This is a model in which a pattern document is the starting point ... – PowerPoint PPT presentation

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Title: XSLT Xpath


1
XSLT Xpath
  • CS196-9

2
High level view
  • We are doing tree editing
  • Changes to node properties
  • Structural rearrangement
  • Several mdels for this kind of task

3
Models for tree editing
  • Functional
  • Rule-based
  • Template-based
  • imperative

4
Functional tree rewriting
  • Recursive processing
  • Invoke start function at the root, construct a
    new tree
  • Can think of this as node functions
  • Result is compositional substitution is
    generally nested
  • Side effects generally avoided caching values,
    clarity.

5
Rule-based (rewriting systems)
  • Idea is that one has a list of patterns
  • Each is a piece of a tree with holes for
    variables.
  • A match leads to replacement of the matched tree
    by a result
  • Variables shared between pattern and result allow
    movement of data
  • Poweful, incremental, definitions
    non-deterministic processing

6
Template based processing
  • This is a model in which a pattern document is
    the starting point
  • It contains literal results interleaved with
    queries and/or imperative code
  • Well-suited to rigid structures
  • Often requires extensions to deal with recursion

7
Imperative
  • Parser calls imperative code, which uses
  • Stacks
  • Global variables
  • Explicit output commands
  • Result is a side effect.
  • Reasoning about the program may be hard, but
    creating it often starts out easily.

8
Whats the big drawback to tree editing?
  • Buffering!
  • You need a copy of the tree to edit
  • This means that its very easy to build
    transformer for a document entirely in-memory
  • Doing this from secondary storage is fairly
    subtle, and has its own performance penalties
  • This is a complex speed/size/coding effort
    tradeoff

9
XSLT falls squarely into the middle of these
approaches
  • Rule-based substitution (but the RHSs look a lot
    like template languages
  • Xpath addressing looks more like the kinds of
    patterns used in traditional template languages
  • Limited non-determinism
  • Sufficient control over rule evaluation order
    that functional transformations are easy
  • No side effects (in standard language)

10
Styles of XSLT transform
  • Functional
  • Rule-based
  • Template-based
  • Imperative

11
Weirdnesses of XPath
  • Navigational language for specifying pattern
    matches
  • You dont match a portion of a tree explicitly,
    rather a node.
  • You can invoke further processing on children
  • You use template-style access functions rather
    than pattern variables

12
Weirdness
  • The language is a mixture of predicate and
    structural pattern
  • Path syntax and functional syntax not a really
    fortunate mix
  • Matching is always relative to a particular node,
    so the first few times results can be very
    puzzling

13
Strategies for XSLT
  • Try to pick a single style as much as possible
  • May vary by project
  • Mixing may be necessary but can get confusing
  • Be sure you understand (and probably override the
    default rules)
  • Shorter patterns are better
  • ltxslvalue-ofgt and ltxslifgt may be easier to deal
    with than a complex path

14
Strategies
  • Use several filters in row
  • Its often easier to manage a series of global
    changes, than interactions between several
    complex conditions.
  • Intermediate results make debugging easier
  • Intermediate results may be cacheable
  • Critical for online applications
  • Where possible code things one element at a time

15
XPath
  • Based on an analogy to Unix path syntax
  • An XPath is composed of steps
  • Each step starts at the context node, and moves
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