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.
来源：https://stackoverflow.com/questions/187414/can-you-explain-lambda-expressions