Interpreter Pattern: Language Translator

The Interpreter Pattern helps your software understand and run its own little language. You build classes for each part of the language, so you can read, write, or change rules, commands, or formulas easily.

This pattern is useful in apps that let users write queries, filters, or formulas. It makes your code easy to update or extend with new features.

Real-Life Analogy: Language Translator

Imagine someone translating simple phrases like “hello” or “thank you” from English to Spanish. The translator knows the rules — how to break down and translate individual words or expressions.

If the phrase fits a pattern, it is interpreted correctly. If not, it is skipped or marked invalid. This is what the Interpreter Pattern does in code — it converts structured input into meaningful output based on rules.

Benefits of Interpreter Pattern

This pattern makes it easier to process expressions or languages that follow a known set of grammar rules. It allows you to add new rules or symbols without rewriting the parser.

It works well when your input is structured and rule-based — such as formulas, filters, or custom scripting.

• Encapsulates grammar rules in easy-to-update classes
• Makes expression trees easy to evaluate or extend
• Good for domain-specific languages (DSLs)

What to Implement

To implement this pattern, you need:

An expression interface, classes that represent grammar rules or symbols, and a method that interprets context.

• Expression Interface: Declares the Interpret method
• Terminal Expression: Represents basic grammar symbols
• Non-Terminal Expression: Combines symbols using logic
• Context: Stores the data to interpret

How It Works in C#

// Context
public class Context
{
    public string Input { get; set; }
    public string Output { get; set; }
}

// Expression Interface
public interface IExpression
{
    void Interpret(Context context);
}

// Terminal Expression
public class HelloExpression : IExpression
{
    public void Interpret(Context context)
    {
        if (context.Input == "hello")
            context.Output = "hola";
    }
}

public class ThanksExpression : IExpression
{
    public void Interpret(Context context)
    {
        if (context.Input == "thank you")
            context.Output = "gracias";
    }
}

Usage:

var context = new Context { Input = "hello" };

List<IExpression> expressions = new()
{
    new HelloExpression(),
    new ThanksExpression()
};

foreach (var expr in expressions)
    expr.Interpret(context);

Console.WriteLine(context.Output); // hola

When Should You Use It?

Use the Interpreter Pattern when your app needs to process structured input, like formulas, search filters, or commands. It's a strong fit when grammar is simple and rarely changes.

Consider it for:

• Text parsing or simple programming languages
• Building rule engines or interpreters
• Filtering logic like search queries or alerts

When Not to Use It: Avoid it if the grammar is complex or changes frequently — it can become hard to maintain.

Where Is It Used in the Real World?

The Interpreter Pattern is used in applications that need to evaluate rules, formulas, or languages. It is common in query languages, filters, and configuration engines.

• SQL and expression evaluators
• Rule engines (workflow, decision tables)
• Search filters (custom query builders)
• Domain-specific languages (DSLs)

Final Thoughts

The Interpreter Pattern helps you break down and process structured input like a mini language. It keeps your grammar rules clean and lets you add new phrases or symbols as needed.

If you're building something that needs rule-based logic, command parsing, or filtering — this pattern can simplify the entire process.