Anonymous Caller Function

You can know about function values but this article also link with my previous blog. Let's start with "Anonymous Caller Function"

package main
import (
"fmt"
)
func squares() func() int {
var x int
return func() int {
x++
return x * x
}
}
func main() {
f := squares()
fmt.Println(f()) // "1"
fmt.Println(f()) // "4"
fmt.Println(f()) // "9"
fmt.Println(f()) // "16"
}  

What we already Know ?
This code confuse you, don't worry let's start what we already know about functions. 
 func squares() int{ return x *x}. This function return square which we pass as argument.
 f := squares() function definition receive by f for further processing.
What we don't know?

func squares() func() int {
var x int
return func() int {
x++
return x * x
}
}

            1. func() int: this is an anonymous function. Anonymous function are those functions which is similar like inner function, but without name. 
           2. return func() int: this function inside square function. Anonymous function use local variables, which are already declare in square function. Return fun() int tells that compiler anonymous function value. Again this is a function so, we can like function except before function return keyword use. Actually Function B take Function A as a input and after processing it return as a output of Function AB. That's why confuse so many times.  

Recursion in Anonymous Function:
             var visitAll func(items []string)
              visitAll = func(items []string) {}
 Recursion in Anonymous function in two steps.
        1. declare a variable along with function definition 
        2. initialize variable with function which to be recursive

func topoSort(m map[string][]string) []string { 

var order []string seen := make(map[string]bool) 

var visitAll func(items []string)

         visitAll = func(items []string) { 
                                   for _, item := range items { 
                                        if !seen[i tem] { seen[item] = true 
                                                visitAll(m[item]) 
                                          order = append(order, item) } 
                                       } 
                                  } 
             var keys []string 
            for key := range m { 
                      keys = append(keys, key)
             }
             sort.Strings(keys)
             visitAll(keys) return order 
            }  
Warning:
If you don't follow these steps these, then program will not execute.