双括号在结构声明中意味着什么？

Question

Can someone please explain me what do double curly brackets {{ mean in this code ? :

func (t *testService) APIs() []rpc.API {
    return []rpc.API{{
        Namespace: "test",
        Version:   "1.0",
        Service: &TestAPI{
            state:     &t.state,
            peerCount: &t.peerCount,
        },
    }}
}

AFIK single curly bracket is enough to create a struct, so why to double it?

the API struct is defined like this:

package rpc

// API describes the set of methods offered over the RPC interface
type API struct {
    Namespace string      // namespace under which the rpc methods of Service are exposed
    Version   string      // api version for DApp's
    Service   interface{} // receiver instance which holds the methods
    Public    bool        // indication if the methods must be considered safe for public use
}

Answer 1

It's a slightly shorter version of this (fewer lines and less indenting):

return []rpc.API{
    {
        Namespace: "test",
        Version:   "1.0",
        Service: &TestAPI{
            state:     &t.state,
            peerCount: &t.peerCount,
        },
    },
}

It's a slice of structs with one element in it.