What's the Purpose of Parentheses Before a Go Function Name?

Function Declaration Syntax in Go: Unraveling the Parentheses

Within Go code, you may encounter function declarations that include parenthesized elements before the function name. These parentheses have a specific role in defining the function's behavior.

The Receiver: An Introduction

The elements enclosed in parentheses represent the receiver for the function. The receiver is the type upon which the method is being invoked. In a sense, it acts as a class in object-oriented programming, representing the entity that the method operates on.

Value and Pointer Receivers

The receiver can either be a value type or a pointer type. Value receivers are passed by value, which means any changes made to the receiver within the function are not reflected in the calling scope. On the other hand, pointer receivers are passed by reference, allowing modifications made within the function to persist after the function returns.

For example, the following function has a value receiver:

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

If the handler variable is modified within the ServeHTTP function (e.g., h.Name = "Evan"), the changes will not be preserved once the function returns.

In contrast, the following function has a pointer receiver:

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

Modifications made to the GracefulServer instance within the BlockingClose function will be reflected in the calling scope.

Parsing the Function Declaration

To fully understand the meaning of a function declaration with a receiver, you need to consider both the receiver type and the function signature.

For instance, the declaration:

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

Indicates that the function ServeHTTP is associated with the handler type and takes two arguments: a http.ResponseWriter and a *http.Request.

Conclusion

The parenthesized elements before the function name in Go function declarations represent the receiver, the type upon which the method is invoked. Receivers can be value or pointer types, and the choice between them depends on whether or not changes to the receiver need to persist in the calling scope.

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