There's a Go issue which is relevant here:
https://github.com/golang/go/issues/8082
On Sat, 25 May 2019 at 04:54, Dan Kortschak <d...@kortschak.io> wrote:
> The interfaces that define the contracts should come from a third
> package/source. The issue that I suspect you are hitting is that type
> identity for interface types is based on the name, not the method set.
> This means that, for example with your code below a function
> PrintB(StringerB) and another function PrintA(StringerA) are not
> assignable to the same variable unless it is type interface{}.
>
> https://play.golang.org/p/a8aWwA6CAB_K // remove comment to see failure
>
> On Fri, 2019-05-24 at 20:38 -0700, Henry wrote:
> > Thanks for the reply.
> >
> > Is there any specific use case that this intended behavior is
> > supposed to
> > solve? It appears to me that it is just a case of simplistic
> > implementation
> > where Go does not look deep enough to see if any dependent interfaces
> > are
> > identical. In this case, Go does not bother to look beyond Formatter
> > interface in order to conclude whether FormatterA and FormatterB are
> > identical. In here, I define 'identical' as being interchangeable,
> > rather
> > than semantic equality. So when I refer to TypeA and TypeB as
> > identical, it
> > means you can use TypeA in place of TypeB, and vice versa.
> >
> > Back to the actual situation, Package A and Package B are developed
> > in
> > tandem by different teams. They have some communication on how their
> > components will interact with each other and fit the bigger picture.
> > They
> > use interface to define contracts that are required in order for
> > their
> > components to work. Since Go interface is implicit, this is supposed
> > to
> > work fine. As an example, if I were to reduce the prior example as
> > follows,
> > it works fine. (Link to the playground:
> > https://play.golang.org/p/zrpjSYTIyxZ)
> >
> > import (
> > "fmt"
> > )
> >
> > func main() {
> > str := MyString("Hello world!")
> > Print(str)
> >
> > strA:=StringerA(str)
> > Print(strA)
> > }
> >
> > type StringerA interface {
> > String() string
> > }
> >
> > type StringerB interface {
> > String() string
> > }
> >
> > type MyString string
> >
> > func (s MyString) String() string {
> > return string(s)
> > }
> >
> > func Print(s StringerB) {
> > fmt.Println(s.String())
> > }
> >
> > However, when I add a more complex interface, as in the first
> > example
> > involving Formatter, it breaks. Go fails to recognize that FormatterA
> > and
> > FormatterB are interchangeable despite the facts that their
> > dependencies
> > (StringerA and StringerB) are interchangeable. Visually, the
> > components
> > should look like this:
> >
> > [image: go.png]
> > Let me know what you think.
> >
> > Thanks.
> >
> > Henry
> >
> > On Saturday, May 25, 2019 at 1:54:11 AM UTC+7, Wagner Riffel wrote:
> > >
> > > It's not a bug, FormatterA and FormatterB method has different
> > > signatures, they are not identical, one wants Format(StringerA)
> > > other
> > > Format(StringerB), thus your Format type only implements
> > > FormatterA
> > > but Print wants a FormatterB.
> > >
> >
> >
>
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