Parse tree design

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

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

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

Possible Solutions

Standard Composite Solution

• 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

• Avoid no-op overrides

Wal's Solution

• 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

• Dependency inversion principle

Wal's Solution Refactored

• 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

• Avoid concrete base classes: Correcting this would add a Lazy class smell

See Also

• Avoid no-op overrides
• Beware type switches
• Decorator
• Refactoring