package main
 
import (
	"fmt"
	"reflect"
)
 
func Map(slice interface{},fn func(a interface{}) interface{}) interface{} {
	val := reflect.ValueOf(slice)
	out := reflect.MakeSlice(reflect.TypeOf(slice),val.Len(),val.Cap())
	for i := 0; i < val.Len(); i++ {
		out.Index(i).Set(
			reflect.ValueOf(fn(val.Index(i).Interface())),)
	}
	return out.Interface()
}
 
func main() {
	a := Map([]int{1,2,3,4},func(val interface{}) interface{} {
		return val.(int) * 2
	})
	fmt.Printf("%T: %v\n",a,a)
}

http://play.golang.org/p/g77ofhvNyj