The interpreter design pattern uses classes to represent grammar rules, allowing the construction of abstract syntax trees (ASTs) for interpreting expressions. Key components include abstract expression classes for literals, alternatives, sequences, and repetitions, making it suitable for simple grammars where efficiency is not critical. While it simplifies grammar implementation and extension, it can be complex to maintain for intricate grammars.

1. Interpreter Design Pattern

2. Introduction to Interpreter Pattern
• The Interpreter pattern uses a class to represent each
grammar rule.
• Abstract syntax trees (ASTs) are built using instances
of these classes to interpret expressions.

5. Key Classes in the Grammar
1. AbstractExpression: Base class for grammar rules.
2. LiteralExpression: Matches literal values in the input.
3. AlternationExpression: Matches one of its alternatives.
4. SequenceExpression: Matches a sequence of
subexpressions.
5. RepetitionExpression: Matches repeated occurrences of
an expression.

6. Abstract Syntax Tree Example
• Expression: raining & (dogs | cats)*
• The AST is built using:
• LiteralExpression for 'raining'.
• AlternationExpression for 'dogs | cats'.
• RepetitionExpression for '*'.

7. Interpret Method in Subclasses
• Each subclass of RegularExpression implements
Interpret:
• - LiteralExpression: Matches the literal value.
• - AlternationExpression: Matches alternatives.
• - RepetitionExpression: Matches repeated
occurrences.

8. Applicability of Interpreter Pattern
• Use this pattern when:
• Language can be represented as an AST
(Abstract Syntax trees)
• Grammar is simple
• Efficiency is not a critical concern

10. Structure - Participants
1. AbstractExpression: Declares the Interpret operation.
2. TerminalExpression: Represents terminal symbols.
3. NonterminalExpression: Represents nonterminal rules.
4. Context: Holds global information for the interpreter.
5. Client: Builds the AST and invokes Interpret.

11. Collaborations
• The Client constructs the AST using
TerminalExpression and NonterminalExpression.
• Context stores interpreter state.
• Interpret operation is recursively called for
subexpressions.

12. Consequences
Benefits:
• Easy to extend/change grammar.
• Simplifies grammar implementation.
Drawbacks:
• Complex grammars are hard to maintain.
• Inefficient for critical applications.

13. Implementation Notes
1. Abstract Syntax Tree creation: Requires a parser or client.
2. Visitor Pattern: Useful for multiple interpretations of expressions.
3. Flyweight Pattern: Optimizes shared terminal symbols.
• Example:
• Each LiteralExpression for 'dog' can share a single
instance.
• Context provides substring and state for matching.