It is called type assertion. Here is how it looks like:
1str := value.(string)
It gives you access to the interface’s concrete value. It asserts if the values stored in x is of type T and that x is not nil.
Type assertion can also return two values: the concrete underlying value and a boolean value that check if the assertion is true.
1t, ok := x.(T)
Here is some code examples for a much easier references. It is a similar example as the one you can find in A Tour of Go
1 var i interface{} = "hello"
2
3 s := i.(string)
4 fmt.Println(s)
5 // Will prints out: hello
6
7 s, ok := i.(string)
8 fmt.Println(s, ok)
9 // Will prints out: hello, true
10
11 f, ok := i.(float64)
12 fmt.Println(f, ok)
13 // Will prints out: 0, false
14
15 f = i.(float64)
16 fmt.Println(f)
17 // Will cause "panic"
18 // panic: interface conversion: interface {} is string, not float64