What exactly does .(data_type) method called/do?

Question

I came a cross a piece of code that used .(string) method. Not knowing what this is called I had difficulties searching for it.

Here is my try to unders

Answer 1

The previous answer is correct. But I submit this as more like what happens in practice. The .(type) syntax is usually used with type names in the cases of a switch. In this example, I (integer expr), B (bool expr), and Bop (binary op) are type names.

func commute (e Expr) (r Expr, d bool) {
    switch exp:= e.(type) {
        case I: r,d = exp,false
        case B: r,d = exp,false
        case Bop: r,d = Bop{exp.op, exp.right, exp.left},true
        default: r,d = e,false
    }
    return
}

This isn't unsafe like a C cast, because inside the case statement you are guaranteed to have the matching type. I see this quite a bit when reading a channel, where the type of the channel is an interface that all the cases implement.