# Closures

A closure is a function that references variables from outside its own function body. The function may access and *assign* to the referenced variables.

In this example, the `concatter()` function returns a function that has reference to an *enclosed* `doc` value. Each successive call to `harryPotterAggregator` mutates that same `doc` variable.

```go
func concatter() func(string) string {
	doc := ""
	return func(word string) string {
		doc += word + " "
		return doc
	}
}

func main() {
	harryPotterAggregator := concatter()
	harryPotterAggregator("Mr.")
	harryPotterAggregator("and")
	harryPotterAggregator("Mrs.")
	harryPotterAggregator("Dursley")
	harryPotterAggregator("of")
	harryPotterAggregator("number")
	harryPotterAggregator("four,")
	harryPotterAggregator("Privet")

	fmt.Println(harryPotterAggregator("Drive"))
	// Mr. and Mrs. Dursley of number four, Privet Drive
}
```

## Assignment

Keeping track of how many emails we send is mission-critical at Mailio. Complete the `adder()` function.

It should return a function that adds its input (an `int`) to an enclosed `sum` value, then return the new sum. In other words, it keeps a running total of the `sum` variable within a closure.