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

itemprop="text">

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 -> href="http://play.golang.org/p/kvw4dcZVNH"
rel="nofollow">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!

Answer

val =
            val.(UnknownMapString)

This
is a type
assertion, which supposes the rel="nofollow">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 href="http://golang.org/ref/spec#Assignability" rel="nofollow">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.

href="http://play.golang.org/p/QyNRwtZPBa"
rel="nofollow">play.golang.org

Output:

test!