The Interpreter pattern is a behavioral design pattern that provides a way to evaluate sentences in a language. It is used to interpret expressions and evaluate grammar or language rules. The pattern defines a grammar for the language and provides an interpreter to interpret sentences in that language.

Key Characteristics:

  • Grammar Definition: Defines the grammar or rules of the language.
  • Interpreter: Provides a way to interpret or evaluate sentences based on the defined grammar.
  • Expression Objects: Encapsulates the interpretation logic for different parts of the grammar.

How It Works:

  • Context: Holds the information that is required for interpreting the expressions.
  • Abstract Expression: Declares an abstract interpret method that will be implemented by concrete expression classes.
  • Terminal Expression: Implements the interpret method for terminal symbols in the grammar.
  • Non-Terminal Expression: Implements the interpret method for non-terminal symbols and uses other expression objects.

Example in PHP: 

  // Context
class Context {
    private $input;

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

    public function getInput() {
        return $this->input;
    }
}

// Abstract Expression
interface Expression {
    public function interpret(Context $context);
}

// Terminal Expression
class TerminalExpression implements Expression {
    private $data;

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

    public function interpret(Context $context) {
        return strpos($context->getInput(), $this->data) !== false;
    }
}

// Non-Terminal Expression
class NonTerminalExpression implements Expression {
    private $expr1;
    private $expr2;

    public function __construct(Expression $expr1, Expression $expr2) {
        $this->expr1 = $expr1;
        $this->expr2 = $expr2;
    }

    public function interpret(Context $context) {
        return $this->expr1->interpret($context) && $this->expr2->interpret($context);
    }
}

// Client Code
$context = new Context("The quick brown fox jumps over the lazy dog");

$expr1 = new TerminalExpression("quick");
$expr2 = new TerminalExpression("dog");
$nonTerminalExpr = new NonTerminalExpression($expr1, $expr2);

if ($nonTerminalExpr->interpret($context)) {
    echo "The context contains both 'quick' and 'dog'.\n";
} else {
    echo "The context does not contain both 'quick' and 'dog'.\n";
}

Explanation:

  1. Context: Context class holds the input that will be interpreted.
  2. Abstract Expression: Expression interface defines the interpret() method that all concrete expressions must implement.
  3. Terminal Expression: TerminalExpression class implements the interpret() method to check if a specific terminal symbol is present in the input.
  4. Non-Terminal Expression: NonTerminalExpression class implements the interpret() method to combine multiple expressions. It uses the AND logical operator in this example.

Benefits:

  • Flexibility: Can interpret complex sentences based on defined grammar rules.
  • Separation of Concerns: Separates the interpretation logic from the sentence structure.
  • Extensibility: New expressions can be added without changing the existing code.

Use Cases:

  • Language Parsing: When creating interpreters or compilers for programming languages.
  • Expression Evaluation: For evaluating complex expressions based on defined rules.
  • Configuration Parsing: Interpreting and processing configuration files or commands.

The Interpreter pattern is useful for creating a language or expression evaluator that adheres to a specific grammar, making it easier to interpret and process sentences or commands in that language.