c# - 为什么 C# 没有词法嵌套函数？

Question

为什么 C# 语言设计者可能不包含对此类内容的支持（来自计算机程序的结构和解释，第二版，第 30 页）：

/// <summary>Return the square root of x.</summary>
double sqrt(double x) {
  bool goodEnough(double guess) {
    return Math.Abs(square(guess) - x) < 0.001;
  }
  double improve(double guess) {
    return average(guess, x / guess);
  }
  double sqrtIter(double guess) {
    return goodEnough(guess) ? guess : sqrtIter(improve(guess));
  }
  sqrtIter(1.0);
}

Answer 1

事实上，C# 正是如此。

double sqrt(double x) {
    var goodEnough = new Func<double, bool>(guess =>
        Math.Abs(square(guess) - x) < 0.001
    );
    var improve = new Func<double, double>(guess =>
        average(guess, x / guess)
    );
    var sqrtIter = default(Func<double, double>);
    sqrtIter = new Func<double, double>(guess =>
        goodEnough(guess) ? guess : sqrtIter(improve(guess))
    );
    return sqrtIter(1.0);
}

Answer 2

正如Justice 所说，您可以使用C# 3.5 和lambdas 来实现；如果你有 C# 2.0，你可以使用匿名函数，虽然它会不那么性感：

double sqrt(double x) {
    Func<double, bool> goodEnough = delegate(double guess) {
        return Math.Abs(square(guess) - x) < 0.001;
    };
    Func<double, double> improve = delegate(double guess) {
        return average(guess, x / guess);
    };
    Func<double, double> sqrtIter = null;
    sqrtIter = delegate(double guess) {
        return goodEnough(guess) ? guess : sqrtIter(improve(guess));
    };
    return sqrtIter(1.0);
}

编辑：我忘了，Func 没有在 C# 2.0 中定义，所以你必须自己定义它：

 public delegate TResult Func<T, TResult>(T guess);