Create method for map[string]interface{} in Go

I have defined a Type

type UnknownMapString map[string]interface{}

I also have methods for them like so

func (m UnknownMapString) Foo() {
    fmt.Println("test!")
}

I get a panic when running:

> interface conversion: interface is map[string]interface {}, not
> main.UnknownMapString

The map[string]interface{} is unmarshaled from JSON input.

Playground replicating it -> http://play.golang.org/p/kvw4dcZVNH

I thought that you could not have interface as a receiver of method so we needed to type assert (not convert?) to a Named Type and use that Named Type as the receiver of the method.
Please let me know what I'm doing wrong. Thanks!

val = val.(UnknownMapString)

This is a type assertion, which supposes the named type UnknownMapString is identical to the unnamed type map[string]interface{}.
And type identity tells us that:

> A named and an unnamed type are always different.

But: you can **assign** a map[string]interface{} to a UnknownMapString because

> x is assignable to a variable of type T ("x is assignable to T") when:

> x's type V and T have identical underlying types and at least one of V or T is not a named type.

This would work:

var val2 UnknownMapString  = val.(map[string]interface{})
val2.Foo()

val2 is not an unnamed type, and both val2 and val.(map[string]interface{}) underlying types are identical.

<kbd>play.golang.org</kbd>

Output:

test!