New to Go. Encountered this error and have had no luck finding the cause or the rationale for it:

If I create a struct, I can obviously assign and re-assign the values no problem:

type Person struct {
 name string
 age int
}

func main() {
  x := Person{"Andy Capp", 98}
  x.age = 99
  fmt.Printf("age: %d\n", x.age)
}

but if the struct is one value in a map:

type Person struct {
     name string
     age int
 }

type People map[string]Person

func main() {
  p := make(People)
  p["HM"] = Person{"Hank McNamara", 39}
  p["HM"].age = p["HM"].age + 1
  fmt.Printf("age: %d\n", p["HM"].age)
}

I get cannot assign to p["HM"].age. That’s it, no other info. http://play.golang.org/p/VRlSItd4eP

I found a way around this – creating an incrementAge func on Person, which can be called and the result assigned to the map key, eg p["HM"] = p["HM"].incrementAge().

But, my question is, what is the reason for this “cannot assign” error, and why shouldn’t I be allowed to assign the struct value directly?