Parse tree design

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A context free grammar defines a language, e.g.

Grammar.gif

A parse tree shows how a sentence in the language is structured according to the grammar.

Parsetree.gif

Contents

Terminology

In parsing lingo a symbol is a name in a grammar. Each symbol is either a terminal or a non-terminal. Non-terminals appear on the left hand side of a grammar production; terminals don't.

Issues Discussed

Around this design we constructed a variety of patterns and discussed them.

Patterns in Use

  • DTSTTCPW Design

Downcasting vs No-ops vs DBC based superclasses

Data Redundancy

Test Driven Development & Refactoring

Possible Solutions

Standard Composite Solution

Parse-tree-sol-basic-composite.png
  • Important methods are declared in the abstract Node class
  • The client can deal entirely with Nodes
  • The client must accept a very loose contract provided by the Node class. For example the add() method may or may not add.
  • The client must check the class type of the object if this contract is insufficient.
  • This design may be suitable if we are only reading from this structure. If this is the case we can interpret the no-ops in Token as meaning Token has no children. From this perspective the no-ops are not necessarily a hack as they semantically make sense.

Violations

Wal's Solution

Parse-tree-sol-wals.png
  • Moves the definition of important methods down to BranchNode to avoid the need for no-ops.
  • The contract defined for the important methods in BranchNode can promise more.
  • Also avoids the need for Downcasting by as*() methods defined in Node
  • This means the Node class must be aware of it's own subclasses.

Violations

Wal's Solution Refactored

Parse-tree-sol-wals-refactored.png
  • Some Tokens don't require the text field, as their text is defined by their type. For example the text of a "SemiColon" Token is always ";".
  • This redundancy is removed by applying Extract Subclass which adds the ValueToken class

Violations

Implementation of grammar

I started thinking about how to use the Flyweight pattern with the implementation for the actual language we developed last Monday. Since I didn't take any notes (duh!) I tried to come up with something be myself but it doesn't really convince me. I recall the diagram on the board being way bigger than mine. However, I see that diagram as a starting point for next class. Maybe there are some correct aspects and at least I learned something about patterns ... :) --TobiW 01:20, 13 August 2009 (UTC)

ParseTreeFlyweight01.png

See Also

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