Can you explain lambda expressions? [duplicate]

问题:

I don't really get lambda expressions. While they've been around since the days of ALGOL, I didn't start hearing about them until fairly recently, when Python and Ruby became very popular. Now that C# has the => syntax, people in my world (.NET) are talking about lamdba expressions more and more.

I've read the Wikipedia article on the lambda calculus, but I'm not really a math guy. I don't really understand it from a practical perspective. When would I use lambda expressions? Why? How would I know that it's what I should be doing?

Can you show examples of how you would solve problems with lambda expressions, in a before-and-after format? Any imperative language is fine, but C# would be easiest for me to understand.

回答1:

Basically as far as C# is concerned, lambda expressions are an easy way to create a delegate (or an expression tree, but let's leave those aside for now).

In C# 1 we could only create delegate instances from normal methods. In C# 2 we gained anonymous methods. In C# 3 we gained lambda expressions, which are like more concise anonymous methods.

They're particularly concise when you want to express some logic which takes one value and returns a value. For instance, in the context of LINQ:

                       // Only include children - a predicate
var query = dataSource.Where(person => person.Age < 18) 
                       // Transform to sequence of names - a projection
                      .Select(person => person.Name);

There's a fuller discussion of this - along with other aspects - in my article on closures.

回答2:

The answers to this question might be useful to you.

回答3:

lambda functions are just anonymous functions.

For example, in python, you want to double all elements in a list. There are pretty much three ways to do so:

List-expressions:

[2*x for x in list]

explicit function:

def double(x):
    return 2*x
map(double, list) # iirc

with lambdas:

double = lambda x : 2*x
map(double, list)

So, a lambda in most languages is just a way to avoid the syntactic overhead of creating a new function.

回答4:

This is one of the better explanations I've seen of how to understand the big ideas in using lambda expressions in C#:

http://www.developingfor.net/c-30/upgrade-your-c-skills-part-3-lambda-expressions.html

回答5:

My main use of lambda expressions in .NET has been when working with lists. Using a lambda expression you can build up a query on a list in a similar way as you would build an SQL statement to search a database table.

回答6:

A lambda expression is a functional value (in the same sense that 'int' and 'string' are values). This means that it is possible to

• define a variable that holds a lambda expression (this is the same as defining a function)
• return a lambda expression from a method or function
• send a lambda expression as an argument to a function or method

Take a look at functions like map, filter or fold (in any functional language) to see how lambda expressions are used as arguments to other functions.