函数声明语法：函数名称前括号内的内容

Question

I'm sorry I couldn't be more specific in the question title, but I was reading some Go code and I encountered function declarations of this form:

func (h handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    ...
}

from https://github.com/mattermost/platform/blob/master/api/context.go

func (s *GracefulServer) BlockingClose() bool {
    ...
}

from https://github.com/braintree/manners/blob/master/server.go

What does the (h handler) and the (s *GracefulServer) between parenthesis mean? What does the entire function declaration mean, taking into account the meaning of the things between parenthesis?

Edit

This is not a duplicate of Whats the difference of functions and methods in Go? : this question came to me because I didn't know what the things in parenthesis before the function name were, not because I wondered what was the difference between functions and methods... if I knew that this declaration was a method I wouldn't have had this question in the first place. If someone has the same doubt as me one day, I don't believe she will go searching for "golang methods" because she doesn't know that this is the case. It would be like wondering what the letter "sigma" means before a mathematical expression (not knowing it means summation) and someone says it's a duplicate of what's the difference between summation and some other thing.

Also, the short answer to this question ("it's a receiver") is no answer to "what's the difference between functions and methods".

Answer 1

This is called the 'receiver'. In the first case (h handler) it is a value type, in the second (s *GracefulServer) it is a pointer. The way this works in Go may vary a bit from some other languages. The receiving type however, works more or less like a class in most object-oriented programming. It is the thing you call the method from, much like if I put some method A in side some class Person then I would need an instance of type Person in order to call A (assuming it's an instance method and not static!).

One gotcha here is that the receiver gets pushed onto the call stack like other arguments so if the receiver is a value type, like in the case of handler then you will be working on a copy of the thing you called the method from meaning something like h.Name = "Evan" would not persist after you return to the calling scope. For this reason anything that expects to change the state of the receiver, needs to use a pointer or return the modified value (gives more of an immutable type paradigm if you're looking for that).

Here's the relevant section from the spec; https://golang.org/ref/spec#Method_sets