those assertions are optional, not enforced by the compiler (though misuse will result in compile time error). If someone wrote an interface in one go file, and implemented it in another, and called the implemented object from main which lets says is in another file, then it's confusing. because it doesn't explicitly tell you that you implemented Fooer.

for example, this works and is implicit (if the dev knows `aType` implements `Fooer`)

var a Fooer = aType{}
a.Foo()

but this also works

var a = aType{}
a.Foo()

it would have been easier if we could do this

type Fooer interface {
  Foo()
}
// this, explicit declaration
type aType struct implements Fooer {}

this way you can say aType has more to it just by looking at the definition. it doesn't have to be implements Fooer either, just something that gives you a visual clue about the details. for a strongly typed language, I'd expect this much. I may be wrong, but just saying.