如何在Golang中间接传递接口

Question

I have a package with a method:

func Route(router *mux.Router){
    subrouter := router.PathPrefix(_API).Subrouter()
    subrouter.Path(_FOO).HandlerFunc(foo)
    subrouter.Path(_BAR).HandlerFunc(bar)
}

and I would like to remove the external dependency of mux by having a matching interface in my package that simple encompasses all the functionality used above, like so:

type Router interface{
    Path(string) Path
    PathPrefix(string) Path
}

type Path interface{
    HandlerFunc(http.HandlerFunc)
    Subrouter() Router
}

func Route(router Router){
    subrouter := router.PathPrefix(_API).Subrouter()
    subrouter.Path(_FOO).HandlerFunc(foo)
    subrouter.Path(_BAR).HandlerFunc(bar)
}

but when I build this I get error:

*mux.Router does not implement api.Router (wrong type for Path method) have Path(string) *mux.Route want Path(string) api.Path

but I thought interfaces were implicitly used in golang so I thought that *mux.Route did implement my Path interface.

Answer 1

I thought interfaces were implicitly used in golang

Values are wrapped in interfaces implicitly, but only in specific cases like when passing an implementation to a function with an interface argument:

func X(api.Path) {}

X(&mux.Route{}) // works, implicitly converted to api.Path

or when returning an implementation from a function with an interface return type:

func Y() api.Path {
    return &mux.Route{} // works, implicitly converted to api.Path
}

In your question's case the compiler wants a value that has a method with the signature:

Path(string) api.Path

But you're giving it a value with a method with signature:

Path(string) *mux.Route

As you might now, Go types are invariant. Formally:

type A interface { Path(string) *mux.Route }

is not a subtype of

type B interface { Path(string) api.Path }

Therefore this won't work.