Interpreter

Interpreter Pattern

Introduction

The Interpreter pattern is a behavioral design pattern that defines a representational grammar for a language and provides an interpreter to deal with this grammar. This pattern is useful for interpreting languages or linguistic constructs, often used in compilers and parsers.

Purpose

• To define a representation for a language’s grammar along with an interpreter that uses this representation to interpret sentences in the language.
• To provide a way to evaluate language grammar or expression.

Use Cases

• When there is a language to interpret, and you can represent statements in the language as abstract syntax trees (AST).
• When the grammar of the language is straightforward and can be easily mapped to a class hierarchy.

Advantages/Disadvantages

Advantages:

• Easy to change and extend the grammar.
• Implementing the grammar is straightforward.

Disadvantages:

• Can get complex and hard to maintain as the grammar becomes more complex.
• Better suited for small, simple grammars.

Implementation in JavaScript

	class AbstractExpression {
	  interpret(context) {}
	}
	
	class TerminalExpression extends AbstractExpression {
	  interpret(context) {
	    console.log('TerminalExpression: Interpret.');
	  }
	}
	
	class NonterminalExpression extends AbstractExpression {
	  interpret(context) {
	    console.log('NonterminalExpression: Interpret.');
	  }
	}
	
	// Usage
	const context = {};
	
	const expressions = [
	  new TerminalExpression(),
	  new NonterminalExpression(),
	  new TerminalExpression(),
	];
	
	expressions.forEach((expression) => expression.interpret(context));

Practical Example

Imagine a scenario where we are building a simple rule engine to process rules defined in a basic language. Each rule can be interpreted to execute a specific action.

	class RuleExpression {
	  interpret(context) {
	    if (context.expression.startsWith('IF')) {
	      console.log('Processing IF condition');
	    }
	  }
	}
	
	class ActionExpression {
	  interpret(context) {
	    if (context.expression.startsWith('ACTION')) {
	      console.log('Executing action');
	    }
	  }
	}
	
	// Usage
	const rule = 'IF some condition ACTION some action';
	const expressions = rule.split(' ').map((expr) => {
	  if (expr === 'IF') {
	    return new RuleExpression();
	  }
	  if (expr === 'ACTION') {
	    return new ActionExpression();
	  }
	});
	
	const context = {};
	expressions.forEach((expr) => {
	  context.expression = expr;
	  expr.interpret(context);
	});

In this example, RuleExpression and ActionExpression are different types of expressions that interpret parts of a simple language. When a rule string is processed, each expression is interpreted accordingly.