interpreter

Interpreter Pattern

Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

Structure

Find below the usual structure of the interpreter pattern:

classDiagram

class Client
class Context
class AbstractExpression
class TerminalExpression
class NonTerminalExpression

Client -->  Context : stores-information
Client -->  AbstractExpression : interacts-with
AbstractExpression <|--  TerminalExpression : implements
AbstractExpression <|--  NonTerminalExpression : implements

Loading

A Client in this structure generally builds (or is given) an AbstractExpression formed with many TerminalExpression and NonTerminalExpression. As expressions are usually flyweights (see Flyweight), a Context is required to store extrinsic information about the expressions. After configuring the context, the client can then execute the Inspect operation on the expression in hand, providing it the execution context needed, the result will be the interpretation performed by the language.

Working example

The working example for this pattern was extracted from GoF's Design Patterns book and is a boolean expression interpreter. It allows for clients to declaratively define complex boolean expressions using subclasses of the BooleanExp interface. See BookExParser for implementation details on what's the project is all about and how the pattern was used in this context.