PHP设计模式之解释器模式的深入解析

/ 
  AbstractExpression. All implementations of this interface 
  are ConcreteExpressions. 
 /
interface MathExpression 
{ 
    / 
     Calculates the value assumed by the expression. 
     Note that $values is passed to all expression but it 
     is used by Variable only. This is required to abstract 
     away the tree structure. 
     /
    public function evaluate(array $values); 
} 

/ 
  A terminal expression which is a literal value. 
 /
class Literal implements MathExpression 
{ 
    private $_value; 

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

    public function evaluate(array $values) 
    { 
        return $this->_value; 
    } 
} 

/ 
  A terminal expression which represents a variable. 
 /
class Variable implements MathExpression 
{ 
    private $_letter; 

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

    public function evaluate(array $values) 
    { 
        return $values[$this->_letter]; 
    } 
} 

/ 
  Nonterminal expression. 
 /
class Sum implements MathExpression 
{ 
    private $_a; 
    private $_b; 

    public function __construct(MathExpression $a, MathExpression $b) 
    { 
        $this->_a = $a; 
        $this->_b = $b; 
    } 

    public function evaluate(array $values) 
    { 
        return $this->_a->evaluate($values) + $this->_b->evaluate($values); 
    } 
} 

/ 
  Nonterminal expression. 
 /
class Product implements MathExpression 
{ 
    private $_a; 
    private $_b; 

    public function __construct(MathExpression $a, MathExpression $b) 
    { 
        $this->_a = $a; 
        $this->_b = $b; 
    } 

    public function evaluate(array $values) 
    { 
        return $this->_a->evaluate($values) $this->_b->evaluate($values); 
    } 
} 

// 10(a + 3) 
$expression = new Product(new Literal(10), new Sum(new Variable('a'), new Literal(3))); 
echo $expression->evaluate(array('a' => 4)), "\n"; 
// adding new rules to the grammar is easy: 
// e.g. Power, Subtraction... 
// thanks to the Composite, manipulation is even simpler: 
// we could add substitute($letter, MathExpression $expr) 
// to the interface...