An interface type-assertion asserts that the dynamic value in the interface
(in this case it's `string`) implements the interface you are asserting (in
this case `myinterface`). As `myinterface` has no methods, every value
implements that interface.
In general, a type-assertion will assert things about the dynamic value
stored in an interface. In that code, the dynamic value is `string` *either
way*. In one case, you assign a `string` to `interface{}`, in the other you
assign a `string` to `myinterface`. But both will ultimately store a
`string`. That's also why your first case, using `mystring` works as
expected - in that case, the dynamic type is `string` in one case and
`mystring` in the other (note that the dynamic type is *always* a "concrete
type", not an interface).
In general, ways to get around this is to either a) not use an interface,
or b) give that interface a method only specific types implement, or c)
store a `*myinterface` - it's a pointer, not an interface, thus a concrete
type, which can be distinguished. However, in your case, neither of these
strategies would likely be fruitful. The `json` package needs to know how
to store the values it decodes and it won't be able to, if it doesn't have
a concrete type or a `json.Unmarshaler`.
So in your specific case, you might want to consider using something like
type struct {
s string
n int64
isString bool
}
and have that implement `json.Marshaler` and `json.Unmarshaler`.
On Tue, Feb 16, 2021 at 8:47 AM Santhosh Kumar T <[email protected]>
wrote:
> I have a function call nextRow which returns []interface{}
>
> i want to add support for json, since json value can be string, number
> which conflicts
> with native types, i used following:
> type Json interface{}
> and returning json values as:
> return Json(v)
> but in type checking it fails.
>
> i extract minimal code to explain my issue:
> https://play.golang.org/p/AAbeOzH-SHE
>
> in the above example, it prints: v3 is myinterface
> but v3 is regular interface{} not of type myinterface
>
> could someone help in resolving the issue.
> Is this expected behavior. what can i do to workaround if so
>
> thanks
> Santhosh
>
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