#Interpreter

#Overview

Define a grammar and an interpreter to evaluate expressions in that language.

#When to use

• You need a small, stable language for rules or filters.
• Expressions are simple and frequent.

#Java example

interface Expression {
    boolean interpret(String context);
}

class ContainsExpression implements Expression {
    private final String token;
    ContainsExpression(String token) { this.token = token; }
    public boolean interpret(String context) {
        return context.contains(token);
    }
}

#TypeScript example

interface Expression {
  interpret(context: string): boolean;
}

class ContainsExpression implements Expression {
  constructor(private token: string) {}
  interpret(context: string): boolean {
    return context.includes(this.token);
  }
}

#Pros and cons

Pros:

• Easy to extend grammar with new expressions.
• Clear representation of rules.

Cons:

• Can become complex for large grammars.
• Performance can degrade with many nodes.

#Common pitfalls

• Using Interpreter for complex DSLs.
• Skipping caching for repeated evaluations.

#Related patterns

• Strategy
• Composite