有什么理由让C＃或Java有lambda？

Question

What reason is there for C# or java having lambdas? Neither language is based around them, it appears to be another coding method to do the same thing that C# already did.
I'm not being confrontational, if there is a reason I would like to know the reason why. For the purpose of full disclosure I am a Java programmer with a C++ background with no lisp experience, I may just be missing the point.

Answer 1

There are common use-cases which require passing (or storing) a block of code to be executed later. The most common would be event listeners. Believe it or not, the following bit of code uses a lambda-ish construct in Java:

JButton button = new JButton("Push me!");
button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        System.out.println("Pressed!");
    }
});

The anonymous inner-class is acting as a lambda, albeit an extremely verbose one. With a little bit of implicit conversion magic, we can write the following equivalent in Scala:

val button = new JButton("Push me!")
button.addActionListener { e =>
  println("Pressed!")
}

C# makes this sort of thing fairly easy with delegates and (even better) lambdas.

Answer 2

I see lambdas in C# as a very convenient short-cut for doing delegates. Much more readable to have the code right there where it is being used rather than having to search elsewhere for the delegate definition.

Answer 3

Syntactic Sugar.

It provides a convenient and more-readable way to represent an idea, in this case a tiny throw-away method. Under the hood, the compiler expands that out to a delegate and method call, but it's the thing doing the work, not you.

Answer 4

Lambdas allow you write less verbose, more expressive code. For example, list comprehensions ...

BTW, work is under way to explore the possibility of adding closures to Java - in the meantime it is necessary to use anonymous classes instead (ugly).

Answer 5

C# isn't going for purity to a particular school of language design (unlike Java, which was designed by the Smalltalkers as to be something of a pure OO language). C# is going for all-things-to-all-people, and it's pretty good at it. C# is based around gathering the best of the various styles of programming into one high-quality, well-supported language. That includes procedural, object-oriented, functional, dynamic, logic, etc. styles of programming. Obviously, so far it doesn't have much in the way of dynamic or logic styles of programming, but that is soon to come (dynamic programming coming with C# 4.0).

Answer 6

In case of C#, lambdas are used internally to implement LINQ. See the article The Evolution Of LINQ And Its Impact On The Design Of C#

Answer 7

Lambda's allow for more readable code in that they allow operations to be defined closer to the point of use rather than like the current C++ method of using function objects whose definition is sometimes far from the point of use. (This is not including some of the boost libraries). I think the key point from lambdas is that they allow more concise and easy to understand code.

Answer 8

They offer better security using the multi threading in Java by implying in many cases the "final" option. So you are not error prone for multitasking.