爲什麼C＃沒有詞彙嵌套函數？

Question

爲什麼會在C＃語言的設計者沒有包括這樣的事情（從Structure and Interpretation of Computer Programs移植，第二版，第30頁。）支持：

/// <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

事實上，C＃有正是這種。

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

Like Justice說，你可以用C＃3.5和lambdas來做到這一點;如果你有C＃2.0中，你可以使用匿名函數，儘管這有些不太性感：

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); 
} 

編輯：我忘了，Func鍵沒有在C＃2.0中定義的，所以你要自己定義：

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