permutation string golang

gistfile1.go

package main

import "fmt"

var arr []string
func main(){
    // array
    arr = make([]string,0)
    force("asd", 3, "", func(thisString string){
        /// array += thisString
        arr = append(arr, thisString)
      //fmt.Println(thisString);
    })
    fmt.Println(arr)
}

func force(str string, n int, thisString string,
           callback func(thisString string)) {
    if n == 0 {
        callback(thisString);
        return;
    }
    
    for _, v := range(str) {
        force(str, n-1, thisString + string(v), callback)
    }
}