Interpreter CS 124 Reference: Gamma et al (“Gang-of-4”), Design Patterns Some material taken from:

Interpreter Intent Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language Motivation Interpreting and evaluating expressions governed by a rules of some language is a common programming problem e.g., arithmetic expressions, regular expressions This pattern provides a way to define the grammar for the language, represent sentences, and then interpret those sentences

Design Solution

Participants Client, context, expression Client typically calls an evaluate method on the context object The call, in turn, calls interpret on several expression objects, and culminates in a returned result The expression objects together represent the entire sentence, and hence are usually contained in another object

Examples Roman numeral parsing: From Simple arithmetic expression parsing: Reverse polish notation From

Example: parsing arithmetic expressions Client (InterpreterDemo) creates Parser object using input sentence Client calls evaluate() on the Parser object Parser contains a context object (a Stack) and a list of Expression objects evaluate() calls interpret(context) on each Expression object Desired result is returned via the context object

Related Patterns The expression structure follows the Composite pattern