[RHSeeger]

Is it possible to indicate, in an interface, that it's not a structural type? Ie, that classes need to specifically indicate they implement it to be considered it's type?

For example, the HtmlString interface could require only "toString" as a method, but not everything that has that method should be considered that type.

[jmpeax]

The idea would be, in this case, to create a method called "toHtmlString". The name "toString" suggests a generic String is created for the object, but HtmlString represents a specially formatted String.

Rather than having a class implement HtmlString as a way to specify that its "toString" method works differently to the usual or expected way for a method of that signature, you can instead write a "toHtmlString" which is more clear. Reading code as "myObject.toString()" vs "myObject.toHtmlString()" is better than having to check the class signature for a special contract pertaining to general-looking method names.

[hjr265]

I would definitely prefer jmpeax's answer.

Although, there was a question/discussion similar to this here: https://www.youtube.com/watch?v=u-kkf76TDHE#t=2584

The gist was that, if the interface and the structs that are supposed to satisfy that interface belong to the same package, and then you want to prevent any outsider from implementing that interface, you can actually include an unexported method:

    type HtmlStringer interface {
        ToString() string
        isHtmlStringer()
    }

Since it is unexported, no outsider struct can actually satisfy this interface anymore.