CODESAMPLE

Interpreter - PHP

Interpreter PHP

The Interpreter pattern defines a way to represent a grammar for a language and provides an interpreter to deal with the sentences in that language. It’s useful when you have a simple language with complex expressions and you want to evaluate those expressions dynamically. This implementation defines a basic calculator language supporting addition and subtraction. Nodes in the grammar (expressions) are represented as classes implementing an interpret() method. The Context class holds variables used during interpretation. The code is designed using simple classes to represent the differing expression types, aligned with PHP’s class-based approach to structure.

<?php

// Context: Stores variables for the interpreter.
class Context {
    private $variables = [];

    public function setVariable(string $name, int $value) {
        $this->variables[$name] = $value;
    }

    public function getVariable(string $name): int {
        return $this->variables[$name] ?? 0; // Return 0 if variable is not set
    }
}

// Expression: Base abstract class for all expressions.
abstract class Expression {
    abstract public function interpret(Context $context): int;
}

// Number: Represents a number in the expression.
class Number extends Expression {
    private $value;

    public function __construct(int $value) {
        $this->value = $value;
    }

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

// Addition: Represents an addition operation.
class Addition extends Expression {
    private $left;
    private $right;

    public function __construct(Expression $left, Expression $right) {
        $this->left = $left;
        $this->right = $right;
    }

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

// Subtraction: Represents a subtraction operation.
class Subtraction extends Expression {
    private $left;
    private $right;

    public function __construct(Expression $left, Expression $right) {
        $this->left = $left;
        $this->right = $right;
    }

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

// Example Usage
$context = new Context();
$context->setVariable('x', 10);
$context->setVariable('y', 5);

$expression = new Addition(new Number(5), new Subtraction(new Number(10), new Number(2)));
$result = $expression->interpret($context);

echo "Result: " . $result . PHP_EOL; // Output: Result: 13

$expression2 = new Subtraction(new Number($context->getVariable('x')), new Number($context->getVariable('y')));
$result2 = $expression2->interpret($context);

echo "Result 2: " . $result2 . PHP_EOL; // Output: Result 2: 5
?>