Is there a way to define a function can run any callback function in Golang?

Question

I want to define a function to run a callback function, and the callback function will be uncertain, such as args or returns both are uncertain.

I try this:

package main

import (
    "fmt"
)

func F(callback func(args ...interface{}) interface{}, args ...interface{}) {
    rev := callback(args...)
    fmt.Println(rev)
}

func CB1() {
    fmt.Println("CB1 called")
}

func CB2() bool {
    fmt.Println("CB2 called")
    return false
}

func CB3(s string) {
    fmt.Println("CB3 called")
}

func CB4(s string) bool {
    fmt.Println("CB4 called")
    return false
}

func main() {
    F(CB1)
    F(CB2)
    F(CB3, "xxx")
    F(CB4, "yyy")
}

errors:

./play.go:31:3: cannot use CB1 (type func()) as type func(...interface {}) interface {} in argument to F
./play.go:32:3: cannot use CB2 (type func() bool) as type func(...interface {}) interface {} in argument to F
./play.go:33:3: cannot use CB3 (type func(string)) as type func(...interface {}) interface {} in argument to F
./play.go:34:3: cannot use CB4 (type func(string) bool) as type func(...interface {}) interface {} in argument to F

This feels like an XY problem. Is there an underlying problem we can help you solve in a more idiomatic Go way? — Duncan Jones
– Duncan Jones, Commented Oct 11, 2018 at 19:52

icza · Accepted Answer · 2018-10-11 12:20:56Z

You can, but since Go does not support generics, you have to define callback to be of type interface{}.

To call a function value stored in callback, you may use reflection, namely the Value.Call() method.

This is how it would look like:

func F(callback interface{}, args ...interface{}) {
    v := reflect.ValueOf(callback)
    if v.Kind() != reflect.Func {
        panic("not a function")
    }
    vargs := make([]reflect.Value, len(args))
    for i, arg := range args {
        vargs[i] = reflect.ValueOf(arg)
    }

    vrets := v.Call(vargs)

    fmt.Print("\tReturn values: ")
    for _, vret := range vrets {
        fmt.Print(vret)
    }
    fmt.Println()
}

With this, the output of your app will be (try it on the Go Playground):

CB1 called
    Return values: 
CB2 called
    Return values: false
CB3 called
    Return values: 
CB4 called
    Return values: false

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