mocked instance method in golang

main.go

package main

import "fmt"

type myType struct {
  fn func(int) int
}

func main () {
  inst := myType{fn: func(i int) int { return i }}
  origFn := inst.fn
  var passedArg int
  inst.fn = func(i int) int {
    passedArg = i
    return origFn(i + 1)
  }
  res := inst.fn(1)
  fmt.Println("fn called with arg %d returned %d", passedArg, res)
  // => "fn called with arg 1 returned 2"
}