Interpreter Design Pattern

Here, we will see how to create Interpreter Design Pattern with example.

Interpreter Design Pattern

The Interpreter Design Pattern is a behavioral design pattern that provides a way to define a language's grammar and an interpreter for that grammar. This pattern is particularly useful when you need to evaluate expressions and rules in a specific language or domain.

Example

A common real-world application of the Interpreter Pattern is in SQL query interpreters. SQL statements can be represented as abstract syntax trees, where each node represents different SQL operations such as SELECT, FROM, WHERE, etc. The interpreter evaluates these trees to execute the queries against a database. We will explain in detail lator part of the article with
code example.

Advantages of the Interpreter Pattern

1. Ease of Adding New Expressions - The structure allows for easily extending the language's grammar by adding new expressions without altering existing code.
2. Clear Separation of Concerns - It maintains a distinct separation between grammar definition and expression evaluation, enhancing code readability.
3. Simplifies Complex Syntax - Complex expressions can be broken down into simpler components, making it easier to interpret them.
4. Reusable Components - Expressions can be reused in different contexts, promoting code reuse.

When to Use the Interpreter Pattern

• Simple Languages - When you have a small, simple language or a set of rules to interpret.
• Dynamic Evaluations - When expressions need to be evaluated dynamically at runtime.
• Grammar Definition - When the language has a well-defined grammar that can be expressed with a combination of simple and complex expressions.

Problems - A System Without an Interpreter

1. Complexity in Parsing - Without an interpreter, parsing and evaluating expressions can lead to convoluted and unmanageable code.
2. Limited Flexibility - Adding new features or rules may require extensive changes to the system, increasing the chance of introducing bugs.
3. Difficult Maintenance - Lack of a structured approach can make maintaining and updating the code challenging, resulting in higher technical debt.

Example of Interpreter - Mathematical Expression

This will evaluate simple mathematical expression. Let's create a simple example where we interpret mathematical expressions consisting of numbers and addition operations.

C# Code Example

This block of code demonstrates the use of the Interpreter Pattern to evaluate a simple mathematical expression like (5 + 3).

using System;

// Abstract Expression
public interface IExpression
{
    int Interpret();
}

// Terminal Expression for Numbers
public class Number : IExpression
{
    private int _number;

    public Number(int number)
    {
        _number = number;
    }

    public int Interpret()
    {
        return _number;
    }
}

// Non-Terminal Expression for Addition
public class Addition : IExpression
{
    private IExpression _leftExpression;
    private IExpression _rightExpression;

    public Addition(IExpression left, IExpression right)
    {
        _leftExpression = left;
        _rightExpression = right;
    }

    public int Interpret()
    {
        return _leftExpression.Interpret() + _rightExpression.Interpret();
    }
}

class Program
{
    static void Main(string[] args)
    {
        // Constructing the expression: (5 + 3)
        IExpression expression = new Addition(new Number(5), new Number(3));

        // Interpreting the expression
        Console.WriteLine($"Result: {expression.Interpret()}"); // Outputs: Result: 8
    }
}

Outtput

Real-World Example - SQL Query Interpreter

A common real-world application of the Interpreter Pattern is in SQL query interpreters. SQL statements can be represented as abstract syntax trees, where each node represents different SQL operations such as SELECT, FROM, WHERE, etc. The interpreter evaluates these trees to execute the queries against a database.

This block of code demonstrates the use of the Interpreter Pattern to generate a simple SQL query such as SELECT Name FROM Employees.

C# Code Example

using System;

// Abstract SQL Expression
public interface ISqlExpression
{
    string Interpret();
}

// Terminal Expression for SELECT
public class Select : ISqlExpression
{
    private string _column;

    public Select(string column)
    {
        _column = column;
    }

    public string Interpret()
    {
        return $"SELECT {_column}";
    }
}

// Terminal Expression for FROM
public class From : ISqlExpression
{
    private string _table;

    public From(string table)
    {
        _table = table;
    }

    public string Interpret()
    {
        return $"FROM {_table}";
    }
}

// Non-Terminal Expression for SQL Query
public class SqlQuery : ISqlExpression
{
    private readonly ISqlExpression _select;
    private readonly ISqlExpression _from;

    public SqlQuery(ISqlExpression select, ISqlExpression from)
    {
        _select = select;
        _from = from;
    }

    public string Interpret()
    {
        return $"{_select.Interpret()} {_from.Interpret()}";
    }
}

class SqlProgram
{
    static void Main(string[] args)
    {
        // Creating SQL Query: SELECT Name FROM Employees
        ISqlExpression select = new Select("Name");
        ISqlExpression from = new From("Employees");
        ISqlExpression query = new SqlQuery(select, from);

        // Interpreting the SQL query
        Console.WriteLine(query.Interpret()); // Outputs: SELECT Name FROM Employees
    }
}

Output

Applications of the Interpreter Design Pattern

1. Expression Languages - Designing interpreters for domain-specific languages (DSLs) where users can define rules and expressions.
2. Compilers and Parsers - Implementing language compilers that need to parse and interpret source code.
3. Configuration Files - Interpreting settings defined in custom formats for applications.
4. Game Development - Evaluating game scripts or rules expressed in a simple language.