Interpreter

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

Introduction

• Interpreter is a design pattern that belongs to the Behavioral Pattern group.

• The Interpreter Pattern helps programmers "build" "dynamic" objects by reading a description of the object and then "building" the object according to that description.

• For example, write a program that allows users to enter the command line according to a pre-specified structure. The program will recognize the Command based on its structure and return appropriate results.

Architecture

Components in the model:

• AbstractionExpression: Declare an interface for performing an operation.

• TerminalExpression: Implements an interpretation operation associated with terminal symbols, acting as a required expression for all terminal symbols in a sentence.

• NonterminalExpression: Can contain an internal TerminalExpression and can also contain another NonterminalExpression. It serves as the “grammar” of the specification language.

• Context : Is the information object to perform interpretation. This object is global to the interpretation process (shared between nodes).

Advantages & disadvantages

Advantage

• Reduce dependency between abstraction and implementation (loose coupling).

• Reduce the number of unnecessary subclasses.

• The code will be cleaner and the application size will be smaller.

• Easier to maintain.

• Easy to expand later.

• Allows you to hide implementation details from the client.

Disadvantage

• The constructed specification language requires a simple grammatical structure.

• Performance is not guaranteed

When to use it

Use Interpreter Pattern when we want:

• Simple grammar set. This pattern needs to define at least one class for each rule in the grammar. Therefore grammars containing many rules can be difficult to manage and maintain.

• Don't care much about performance. Because the grammar is analyzed in a hierarchical structure (tree), performance is not guaranteed.

• The Interpreter Pattern is often used in compilers, defining grammars, rules, SQL, XML parsers, etc.

Related Design Patterns

• The abstract syntax tree is an implementation of the Composite pattern .

• Interpreters often use an Iterator to traverse the structure.

• Visitor can be used to maintain behavior on each node in the class's abstract syntax tree.