Function types in Go - particular type casting to more general type -

what cast / assertion need in go in order pass function expecting generic function func(interface{}) interface{}, more specific function func(int) int instead?

for example, in code this, fooa can passed makeexclamer, not foob:

func makeexclamer(foo func (interface{}) interface{}, n int) func () {     return func() {         fmt.printf("%v!!!", foo(n))     } }  func fooa(x interface{}) interface{} {     return x.(int)*2 }  func foob(x int) int {     return x * 10 }  func main() {     exclamera := makeexclamer(fooa, 12)     exclamera()     exclamerb := makeexclamer(foob, 66) // >> cannot use foob (type func(int) int) type func(interface {}) interface {} in argument makeexclamer      exclamerb() } 

(go playground link: https://play.golang.org/p/xgzfco0iag)

i'm not interested in alternative code structure patterns, since how want work: specific function should passed general function transformer (accepting function of type any -> any) return general function (any -> any). may not idiomatic in go, pattern want code follow.

to use type assertions, every possible type must enumerated in makeexclamer:

func makeexclamer(fn interface{}, arg interface{}) func() {     switch fn := fn.(type) {     case func(int) int:         return func() {             fmt.printf("%v!!!\n", fn(arg.(int)))         }     case func(interface{}) interface{}:         return func() {             fmt.printf("%v!!!\n", fn(arg))         }     default:         panic("not supported")     } } 

to accept function of type, fn argument declared type interface{}. code uses type switch handle different function types.

playground example

reflection can used write more general function.

func makeexclamer(fn interface{}, arg interface{}) func() {     fnr := reflect.valueof(fn)     argr := reflect.valueof(arg)     return func() {         resultr := fnr.call([]reflect.value{argr})         fmt.printf("%v!!!\n", resultr[0].interface())     } } 

playground example