How to understand this line in the code?

Question

go

I'm looking at the documentation for the sessions module for martini here . And I don't quite understand this part:

v := session.Get("hello")
return v.(string)

v is a structure as I understand it. To refer to a field, I have to write v.name (for example). To call a method, I have to do v.name(), but what does v.(string) mean?

Answer 1

v is an empty interface {} interface, which can contain absolutely any value of any data type. To get this value from there and use it, you need to cast it to the desired type.
v.(string) casts it to string. We can do this here, because we know that Get returns a string to us. If v does not contain a string, we will get a panic on the v.(string) line.
https://tour.golang.org/methods/15