Investigate how to forward as an interface

[kdungs]

E.g.

f := result.Wrap(os.Open("foo"))
result.MapE(io.ReadAll)

doesn't compile because in MapE we have B = *os.File which implements io.Reader but is not identical to it...

[kdungs]

Doesn't work because among other things https://go.dev/doc/faq#covariant_types.

//edit: The reason I still think this is more of a generics issue than a covariant types issue is that the following works:

func c[A any, B interface{ io.Reader }](f func(A) B, g func(io.Reader) string) func(A) string {
	return func(a A) string {
		return g(f(a))
	}
}

but the rules for constraints currently prohibit using a type parameter inside interface {}.

[kdungs]

See also golang/go#58650

[kdungs]

Collected examples from the discussion on the Gopher slack...

• https://go.dev/play/p/2G4qPqK_2-T
• https://go.dev/play/p/EK6B2FOUwJ1