CODEWITHSUNDEEP
functiongoclosures
s666
s666

Reputation: 274

Need more comments on Function Closures of Go

Below is the code of Function Closures in A Tour of Go, I know a little about function closures but I am a beginner for Go.

package main

import "fmt"

func adder() func(int) int {
    sum := 0
    return func(x int) int {
        sum += x
        return sum
    }
}

func main() {
    pos, neg := adder(), adder()
    for i := 0; i < 10; i++ {
        fmt.Println(
            pos(i),
            neg(-2*i),
        )
    }
}

Here are questions:

  1. for func adder() func(int) int { what are [parameter list] and [return_types] respectively?
  2. for the same line with Question.1, why there is (int), instead of something like (x int)?
  3. for pos, neg := adder(), adder(), does it mean to assign the function adder to pos and neg, why not it should be pos, neg := adder, adder?

Upvotes: 2

Views: 217

Answers (1)

Levan
Levan

Reputation: 676

Q: for func adder() func(int) int { what are [parameter list] and [return_types] respectively?

A: Here, we have a function called adder() that takes no arguments that returns a function func(int) int which takes an integer and returns an integer.

Q: for the same line with Question.1, why there is (int), instead of something like (x int)?

A: This is the adder() function

func adder() func(int) int {
    sum := 0
    return func(x int) int {
        sum += x
        return sum
    }
}

Take a look at the function that is being returned, here func(x int) int already has a named parameter (x), so we don't need to mention it again at func adder() func(int) int, because if we'd do something like func adder() func(x int) int, the x here has no use.

So if the function that is being returned had, say, 2 parameters (one extra is a string type), then it would look like the following code:

func adder() func(int, string) int {
    sum := 0
    return func(x int, y string) int {
        sum += x
        fmt.Println(y)
        return sum
    }
}

Notice that we added a string type at func adder() func(int, string) int {, that's because the function we are returning takes a string type.

Q: for pos, neg := adder(), adder(), does it mean to assign the function adder to pos and neg, why not it should be pos, neg := adder, adder?

A: See, when we assign pos to adder() (pos := adder()), pos becomes a new function, because adder() returned a function func(int) int, thus we can do pos(i).

Same goes for neg.

Upvotes: 3

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