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...