LINQ的恶搞……

在去年QCon SF的一个访谈里，Erik Meijer提到可以把这样一个lambda表达式:

x => x + 1

写成：

from x in default(int)
select x + 1

原话是：

Erik Meijer 写道

In my talk on LINQ, I have an example of a really weird implementation that shows that you don't need lambda expressions. If you implement the sequence operator correctly you can say "from x in default(int) select x + 1" and that means "x => x + 1". You can do weird things like that, but again, I think we are very early in the development and when people will discover this, that LINQ is not just for collections, you will see many other applications of it.

怪有趣（和无聊=w=）的。
实践一下，代码如下：

using System;

static class Int32Extensions {
    public static Func<int, TR> Select<TR>(this int i, Func<int, TR> func) {
        return func;
    }
}

static class Program {
    static void Main(string[] args) {
        var f = from x in default(int)
                select x + 1;
        Console.WriteLine(f.GetType()); // System.Func`2[System.Int32,System.Int32]
        Console.WriteLine(f(2));        // 3
    }
}

连System.Linq都不需要引用，确实能行。呵呵，Erik真欢乐 XDD

话说这代码也能泛化到任意类型：

using System;

static class TExtensions {
    public static Func<T, TR> Select<T, TR>(this T obj, Func<T, TR> func) {
        return func;
    }
}

static class Program {
    static void Main(string[] args) {
        var f = from s in default(string)
                select s.Length;
        Console.WriteLine(f.GetType()); // System.Func`2[System.String,System.Int32]
        Console.WriteLine(f("oh my"));  // 5
    }
}

太邪恶了……（虽然函数不能像F#那样自动泛化比较不爽 =v=）
话说泛化到任意类型之后，编译会得到一个警告：

引用

warning CS1720: Expression will always cause a System.NullReferenceException because the default value of 'string' is null

不过实际上没关系，我们根本就没用过那个null。

======================================================

糟糕，刚才不小心点了提交，直接就发出来了……发的时候标题跟内容是脱节的 OTL
嘛，LINQ的语法是monadic syntax没错，不过跟这帖没关系 =v=