Go Advice: Use Interface assertion declarations

In Go, interfaces are implicitly implemented by types. The type checker verifies that a value has a type which correctly implements the interface at the places where the value is being used and not where its type is declared¹. Sometimes, it’s nice to have earlier proof than this.

A simple but effective pattern in Go is to assert the type conversion explicitly within a declaration so that you know that the interface has been implemented correctly. I like to do this because it also works as documentation, and it works early in the process of implementing a new type before you know exactly where it will be used. It also helps prove whether the type implements the interface via the value or the pointer to a value.

type Point struct {X, Y int }
 
var _ fmt.Stringer = (*Point)(nil) // ./prog.go:7:22: cannot use (*Point)(nil) (value of type *Point) as fmt.Stringer value in variable declaration: *Point does not implement fmt.Stringer (missing method String)
 

In the code above, it looks like we intended to implement the String method on the Point type, but since we didn’t, we get a compiler error. I observe a few things about this approach of using an “interface assertion declaration” to check this early:

1. Notice the declaration uses a discard for its name. Since we don’t actually care about ever referencing this value again, we can just use _ for its name.
2. Notice the conversion of an untyped nil literal to a typed nil value. This is a cute way to avoid making an allocation (i.e. new(Point))

Interestingly, we decided to make Point implement Stringer via a pointer instead of directly. Does this have any implications?

Well, it turns out that with this declaration, we can implement this interface using either selector for Point:

 
// Either of these will satisfy the interface!
func (Point) String() string { ... }
func (*Point) String() string { ... }
 

However, if you assert the non-pointer type in the interface assertion declaration, then the selector must also use the non-pointer type and using the pointer type is invalid. (See what I mean - Go Play).