Go中的函数类型-将特定类型转换为更通用的类型

Question

What cast / assertion need I do in Go in order to pass to a function expecting a generic function like func(interface{}) interface{}, a more specific function like func(int) int instead?

For example, in code like this, fooA can be passed to MakeExclamer, but 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) as type func(interface {}) interface {} in argument to MakeExclamer 
    exclamerB()
}

(Go Playground link: https://play.golang.org/p/xGzfco0IAG)

I'm not interested much in alternative code structure patterns, since this is how I want it to work: a specific function should be passed to a general function transformer (accepting function of type Any -> Any) that will return another general function (Any -> Any). This may not be idiomatic in Go, but it is the pattern that I want my code to follow.

Answer 1

To use type assertions, every possible type must be enumerated in MakeExclamer:

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

To accept a function of any type, the fn argument is declared as type interface{}. The code uses a type switch to handle the different function types.

playground example

Reflection can be used to write a 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!!!
", resultr[0].Interface())
    }
}

playground example