Denotate Peano's integer addition using Golang - Kind of pattern matching based on types only

peano's integer

package main

import "fmt"

type Peano interface { isPeano() }

// Zero
type Zero struct {}
func (z Zero) isPeano() {}

// Succ
type Succ struct { n Peano }
func (s Succ) isPeano() {}

func eval(p Peano) int {
   switch e := p.(type) {
      case Zero: return 0
      case Succ: return 1 + eval(e.n)
   }
   return 0;
}

func main() {
    fmt.Println(eval(Succ{Succ{Zero{}}}))
}