It seems to me that the compiling error is adequate guidance.
fmt.Printf("%v %T",V.Calculate(2,8)) displays 10 %!T(MISSING)
ie Calculate returns a untyped value.
If you try to set the type by using the standard interface of type
CalcB fmt.Println(ib.Calculate(CalcB(i)).Result()) then the compiling error
is
prog.go:40: cannot convert i (type int) to type CalcB:
int does not implement CalcB (missing Calculate method)
prog.go:40: cannot use CalcB(i) (type CalcB) as type int in argument to
ib.Calculate
which confirms that Calculate method typed as ResultA fails probably because
the compiler considers unclear what should be achieved.
On Monday, July 4, 2016 at 5:29:32 AM UTC+2, Ian Lance Taylor wrote:
>
> On Sun, Jul 3, 2016 at 12:27 PM, Jon Bodner <j...@bodnerfamily.com
> <javascript:>> wrote:
> >
> > I'm working on a DAO adapter layer helper called Proteus
> > (https://github.com/jonbodner/proteus), and ran into an interesting
> quirk in
> > what Go considers equivalent interfaces. I recreated a simplified
> version of
> > the issue is at https://play.golang.org/p/BS3-bUKtO9 .
> >
> > The code that I linked does not compile, with the error:
> >
> > V does not implement CalcB (wrong type for Calculate method)
> > have Calculate(int) ResultA
> > want Calculate(int) ResultB
> >
> >
> > ResultA and ResultB are identical interfaces; both define a single
> method
> > Result that takes in no parameters and return an int.
> >
> > However, if I put in a wrapper around V, I can get it to work:
> > https://play.golang.org/p/XUoZwsBaFn
> >
> > What I don't understand is that the wrapper doesn't do any casting of
> > ResultA to ResultB, and that's OK in this situation. But the Go compiler
> > can't figure out that ResultA and ResultB are identical without that
> > wrapper. Why is that? Why does the compiler fail the first case and pass
> the
> > second one?
>
> ResultA and ResultB are assignable but not identical.
>
> https://golang.org/ref/spec#Type_identity
> https://golang.org/ref/spec#Assignability
>
> For the specific case of interface types, you may be interested in
> https://golang.org/issue/16209 .
>
> Ian
>
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