Meaning of underscore (blank identifier) in Go [duplicate]

Question

As I was reading the Go docs I found this:

You can ask the compiler to check that the type T implements the interface I by attempting an assignment:

type T struct{} var _ I = T{} // Verify that T implements I. 

I don't understand what the _ is used for and I've seen it in other assignments but cannot understand what it means. Digging deeper I found that it's called "blank identifier" but I don't understand the use case they put:

_ = x // evaluate x but ignore it 

Go idioms still feel a little alien to me so I'm trying to understand why I would want to do something like this.

Answer 1

_ is a special identifier you can assign anything to but never read from. In the first example you gave:

var _ I = T{} 

There is no way to access this variable so it will be optimised out of the resulting program. However, it could cause a compile error if the type T is not assignable to the interface I. So in this case it is being used as a static assertion about a type.

The second case is more common. While it might seem strange to throw away the result of a function call, it can make more sense in functions with multiple returns. Consider a function foo that returns two values but you're only interested in the first? You can use _ to ignore the second:

a, _ = foo() 

You could get the same effect by creating another variable to hold the unwanted return value, but this feature means you don't need to worry about picking a unique name for it.