Invoke a function which is received as an interface variable in golang

Question

I have a code which is similar to the following

package main

import "fmt"

func PrintThis(arg string) {
    fmt.Printf("I'm printing %s", arg)
}

func PrintThisAndThat(arg1, arg2 string) {
    fmt.Printf("Now printing %s and %s", arg1, arg2)
}

func Invoke(fn interface{}, args ...string) {
    //fn(args...)
}

func main() {
    Invoke(PrintThis, "foo")
    Invoke(PrintThisAndThat, "foo", "bar")
}

This is not the actual production code, but this is a simplified version.

Question :- If I uncomment the line //fn(args...) I get a compile error prog.go:14: cannot call non-function fn (type interface {})

How do I execute the function which is received as the argument tho the Invoke() function?

What is the right way to achieve this?

Answer 1

You use the Call or CallSlice methods of the reflect.Value to call it as a function. As with all reflect.Value methods, this panics is fn is the wrong type.

func Invoke(fn interface{}, args ...string) {
    v := reflect.ValueOf(fn)
    rargs := make([]reflect.Value, len(args))
    for i, a := range args {
        rargs[i] = reflect.ValueOf(a)
    }
    v.Call(rargs)
}

http://play.golang.org/p/xGmNLDcLL_

Answer 2

You can use a type switch like so http://play.golang.org/p/opotbIGdrA

package main

import "fmt"

func PrintThis(arg string) {
    fmt.Printf("I'm printing %s", arg)
}

func PrintThisAndThat(arg1, arg2 string) {
    fmt.Printf("Now printing %s and %s", arg1, arg2)
}

func Invoke(fn interface{}, args ...string) {
    switch m := fn.(type) {
    case func(string):
        m(args[0])
    case func(string, string):
        m(args[0], args[1])
    default:

    }
}

func main() {
    Invoke(PrintThis, "foo")
    Invoke(PrintThisAndThat, "foo", "bar")
}

But you kind of need to know what functions will be passed to make this work properly.

Btw you can turn PrintThis into a variadic function by using ...string instead of multiple string arguments.