The Go Programming Language

7.15 A Few Words of Advice

When designing a new package, novice Go programmers often start by creating a set of interfaces and only later define the concrete types that satisfy them. This approach results in many interfaces, each of which has only a single implementation. Don’t do that. Such interfaces are unnecessary abstractions; they also have a run-time cost. You can restrict which methods of a type or fields of a struct are visible outside a package using the export mechanism (§6.6). Interfaces are only needed when there are two or more concrete types that must be dealt with in a uniform way.

We make an exception to this rule when an interface is satisfied by a single concrete type but that type cannot live in the same package as the ...