The Go Programming Language

7.13 Type Switches

Interfaces are used in two distinct styles. In the first style, exemplified by io.Reader, io.Writer, fmt.Stringer, sort.Interface, http.Handler, and error, an interface’s methods express the similarities of the concrete types that satisfy the interface but hide the representation details and intrinsic operations of those concrete types. The emphasis is on the methods, not on the concrete types.

The second style exploits the ability of an interface value to hold values of a variety of concrete types and considers the interface to be the union of those types. Type assertions are used to discriminate among these types dynamically and treat each case differently. In this style, the emphasis is on the concrete types that satisfy ...