External.go

func main() {
    t := NewThing(0)
    for i := 0; i < 10; i++ {
        inc(t)
    }
}

func addOne(i int) {
    return i + 1
}

func inc(t Thing) int{
    c := make(chan Message)
    t.external <- Message{addOne, c}
    return <- c
}

Internal.go

type Thing struct {
    state    int
    external chan message
}

type Message struct {
    f   func(int) int
    ret chan int
}

func NewThing(i int) Thing{
    t := new(Thing)
    t.state = i
    t.external = make(chan Message)
    go t.run()
    return t
}

func (t Thing) run() {
    for m := range t.external {
        t.i = m.f(t.i)
        m.ret <- t.i
    }
}