结构

解释器模式

说明

Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

给定一个语言, 定义它的文法的一种表示,并定义一个解释器, 该解释器使用该表示来解释语言中的句子。

适用条件

当有一个语言需要解释执行, 并且可以将语言中的句子表示为一个抽象的表达式树时, 可以使用解释器模式。

实现

逆波兰表示法为例, 其语法为:

expression ::= plus | minus | variable | number
plus ::= expression expression '+'
minus ::= expression expression '-'
variable  ::= 'a' | 'b' | 'c' | ... | 'z'
digit = '0' | '1' | ... '9'
number ::= digit | digit number

波兰表示法的例子:

a b +
a b c + -
a b + c a - -

以下是逆波兰表示法的解释器实现 :

public interface IExpression {

   int Interpret(Dictionary<string, IExpression> variables);

}

public class Number : IExpression {

   private readonly int _number;

   public Number(int number) {
      this._number = number;
   }

   public int Interpret(Dictionary<string, IExpression> variables) {
      return this._number;
   }

}

public class Variable : IExpression {

   private readonly string _name;

   public Variable(string name) {
      this._name = name;
   }

   public int Interpret(Dictionary<string, IExpression> variables) {
      if (string.IsNullOrEmpty(this._name)) {
         return 0;
      }
      return variables[this._name].Interpret(variables);
   }
}

public class Plus : IExpression {

   private readonly IExpression _leftOperand;
   private readonly IExpression _rightOperand;

   public Plus(IExpression leftOperand, IExpression rightOperand) {
      this._leftOperand = leftOperand;
      this._rightOperand = rightOperand;
   }

   public int Interpret(Dictionary<string, IExpression> variables) {
      return this._leftOperand.Interpret(variables) + this._rightOperand.Interpret(variables);
   }

}

public class Minus : IExpression {
   
   private readonly IExpression _leftOperand;
   private readonly IExpression _rightOperand;

   public Minus(IExpression leftOperand, IExpression rightOperand) {
      this._leftOperand = leftOperand;
      this._rightOperand = rightOperand;
   }

   public int Interpret(Dictionary<string, IExpression> variables) {
      return this._leftOperand.Interpret(variables) - this._rightOperand.Interpret(variables);
   }

}

public class Evaluator : IExpression {

   private readonly IExpression _expressionTree;

   public Evaluator(string expression) {
      var stack = new Stack<IExpression>();
      foreach (var token in expression.Split(' ')) {
         if (token == "+") {
            stack.Push(new Plus(stack.Pop(), stack.Pop()));
         }
         else if (token == "-") {
            var right = stack.Pop();
            var left = stack.Pop();
            stack.Push(new Minus(left, right));
         }
         else {
            stack.Push(new Variable(token));
         }
      }
      this._expressionTree = stack.Pop();
   }
   
   public int Interpret(Dictionary<string, IExpression> variables) {
      return this._expressionTree.Interpret(variables);
   }
}


class Client {

   static void Main(string[] args) {
      const string expression = "w x z - +";
      var evaluator = new Evaluator(expression);
      var sentence = new Dictionary<string, IExpression>();
      sentence["w"] = new Number(5);
      sentence["x"] = new Number(10);
      sentence["z"] = new Number(42);
      var result = evaluator.Interpret(sentence);
      Console.WriteLine(result);

      Console.ReadKey();
   }
}