Hello, fellow developers!🧑🏼‍💻

In the realm of software development, design patterns play a pivotal role in crafting efficient and scalable applications. One such pattern, the Interpreter pattern, offers a unique approach to evaluating language grammar or expressions for specific languages. Essentially, it's akin to building a mini language processor. In this article, we will delve into the intricacies of the Interpreter pattern and demonstrate its implementation in PHP 8 through the creation of a simple mathematical expression evaluator.

Understanding the Interpreter Pattern

The Interpreter pattern is designed to interpret sentences in a language. It involves defining a representation for the language's grammar alongside an interpreter to process this grammar. This pattern is particularly useful when you need to design a tool that interprets or compiles languages, offering a structured approach to translating one form of data into another.

Before we dive into the coding part, let's understand the components involved in this pattern:

Expression Interface: This is a blueprint for our expressions, defining a method that will interpret different kinds of expressions.
Terminal Expressions: These are the primary expressions that return a value.
Non-Terminal Expressions: These expressions represent operations and use terminal expressions to complete their tasks.

With this understanding, let's proceed to build our mathematical expression evaluator.

Building a Simple Mathematical Expression Evaluator

Step 1: Expression Interface

First, we define an interface for our expressions, which declares a method to interpret the expressions:

interface Expression
{
    public function interpret(array $context): int;
}

Step 2: Terminal Expressions

Next, we create the main expressions that return a value. These expressions represent the numbers in our mathematical expressions:

class Number implements Expression
{
    public function __construct(private int $number) {}

    public function interpret(array $context): int
    {
        return $this->number;
    }
}

Step 3: Non-Terminal Expressions

Now, we develop expressions that represent operations, utilizing terminal expressions to return a result:

class Add implements Expression
{
    public function __construct(private Expression $left, private Expression $right) {}

    public function interpret(array $context): int
    {
        return $this->left->interpret($context) + $this->right->interpret($context);
    }
}

class Subtract implements Expression
{
    public function __construct(private Expression $left, private Expression $right) {}

    public function interpret(array $context): int
    {
        return $this->left->interpret($context) - $this->right->interpret($context);
    }
}

Step 4: Client Code

Finally, we create a client code to interpret and evaluate mathematical expressions:

class Client
{
    public static function main()
    {
        $context = [];

        // Creating number expressions
        $five = new Number(5);
        $three = new Number(3);

        // Creating and interpreting add expression
        $addExpression = new Add($five, $three);
        echo "Result of addition: " . $addExpression->interpret($context) . "\n"; // Output: 8

        // Creating and interpreting subtract expression
        $subtractExpression = new Subtract($five, $three);
        echo "Result of subtraction: " . $subtractExpression->interpret($context) . "\n"; // Output: 2
    }
}

// Running the client code
Client::main();

In this client code, we create Number objects representing the numbers 5 and 3, and then create Add and Subtract expressions using these numbers. We interpret these expressions to get the results, which are then printed.

Through this article, we have explored the Interpreter pattern and its practical implementation in PHP 8 by building a simple mathematical expression evaluator. This pattern offers a structured approach to designing language compilers or interpreters, showcasing the power and flexibility of object-oriented programming.

As you venture further into the world of design patterns, you'll find that the Interpreter pattern can be a valuable tool in your developer toolkit, especially when dealing with language processing tasks. Happy coding!

References

Design Patterns: Elements of Reusable Object-Oriented Software - Book by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides
PHP 8 Documentation - Official PHP 8 Documentation

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Design Patterns in PHP 8: Interpreter