15-closure.md

This Go program demonstrates the use of closures by defining a function that returns another function.

// Importing the "fmt" package, which provides functions for formatted I/O.
import "fmt"

// Function intSeq returns a closure: a function that "closes over" the variable i.
// The returned function can be used to generate a sequence of increasing integers.
func intSeq() func() int {
    // Initialize a variable i.
    i := 0

    // Define and return an anonymous function (closure) that increments and returns i.
    return func() int {
        i++
        return i
    }
}

// The main function, which serves as the entry point for the program.
func main() {
    // Call intSeq, which returns a function that will generate a sequence of integers.
    nextInt := intSeq()

    // Call the returned function, printing the next value in the sequence.
    fmt.Println(nextInt())

    // Call the returned function again, printing the next value in the sequence.
    fmt.Println(nextInt())

    // Call the returned function once more, printing the next value in the sequence.
    fmt.Println(nextInt())

    // Call intSeq again, creating a new closure with its own "i" variable.
    newInts := intSeq()

    // Call the new closure, printing the first value in its own sequence.
    fmt.Println(newInts())
}

Output

1
2
3
1

Now, let's break down the code and explain each part:

1. Function Declarations:

   • intSeq() func() int: This function returns another function (a closure) that generates a sequence of increasing integers. The returned function "closes over" the variable i.

2. Main Function:

   • main(): This is the entry point of the program.

   • nextInt := intSeq(): Calls intSeq, which returns a closure (a function that increments and returns i). The closure is assigned to the variable nextInt.

   • fmt.Println(nextInt()): Calls the closure (stored in nextInt), printing the next value in the sequence.

   • fmt.Println(nextInt()): Calls the closure again, printing the next value in the sequence.

   • fmt.Println(nextInt()): Calls the closure once more, printing the next value in the sequence.

   • newInts := intSeq(): Calls intSeq again, creating a new closure with its own i variable.

   • fmt.Println(newInts()): Calls the new closure, printing the first value in its own sequence.