Referring to the answer of jnml, you could prevent new instances of Base type by not exporting the Base type at all (i.e. write it lowercase). If needed, you may make an exportable interface that has a method that returns a base type. This interface could be used in functions from the outside that deal with Bases, i.e.
package a
type base int
const (
A base = iota
C
T
G
)
type Baser interface {
Base() base
}
// every base must fulfill the Baser interface
func(b base) Base() base {
return b
}
func(b base) OtherMethod() {
}
package main
import "a"
// func from the outside that handles a.base via a.Baser
// since a.base is not exported, only exported bases that are created within package a may be used, like a.A, a.C, a.T. and a.G
func HandleBasers(b a.Baser) {
base := b.Base()
base.OtherMethod()
}
// func from the outside that returns a.A or a.C, depending of condition
func AorC(condition bool) a.Baser {
if condition {
return a.A
}
return a.C
}
Inside the main package a.Baser is effectively like an enum now.
Only inside the a package you may define new instances.