What is "_," (underscore comma) in a Go declaration?

And I can't seem to understand this kind of variable declaration:

_, prs := m["example"]

What exactly is "_," doing and why have they declared a variable like this instead of

prs := m["example"]

(I found it as part of Go by Example: Maps)

It avoids having to declare all the variables for the returns values.
It is called the blank identifier.

As in:

_, y, _ := coord(p)  // coord() returns three values; only interested in y coordinate

That way, you don't have to declare a variable you won't use: Go would not allow it. Instead, use '_' to ignore said variable.

(the other '_' use case is for import)

Since it discards the return value, it is helpful when you want to check only one of the returned values, as in "How to test key existence in a map?" shown in "Effective Go, map":

_, present := timeZone[tz]

> To test for presence in the map without worrying about the actual value, you can use the blank identifier, a simple underscore (_).
The blank identifier can be assigned or declared with any value of any type, with the value discarded harmlessly.
For testing presence in a map, use the blank identifier in place of the usual variable for the value.

As Jsor adds in the comments:

> "generally accepted standard" is to call the membership test variables "ok" (same for checking if a channel read was valid or not)

That allows you to combine it with test:

if _, err := os.Stat(path); os.IsNotExist(err) {
	fmt.Printf("%s does not exist\n", path)
}

You would find it also in loop:

> If you only need the second item in the range (the value), use the blank identifier, an underscore, to discard the first:

sum := 0
for _, value := range array {
    sum += value
}

The Go compiler won't allow you to create variables that you never use.

for i, value := range x {
   total += value
}

The above code will return an error message "i declared and not used".

Since we don't use i inside of our loop we need to change it to this:

for _, value := range x {
   total += value
}

>The blank identifier may be used whenever syntax requires a variable name but program logic does not, for instance to discard an unwanted loop index when we require only the element value.

Excerpt From:

The Go Programming Language (Addison-Wesley Professional Computing Series)

Brian W. Kernighan

This material may be protected by copyright.

_ is the blank identifier. Meaning the value it should be assigned is discarded.

Here it is the value of example key that is discarded. The second line of code would discard the presence boolean and store the value in prs.
So to only check the presence in the map, you can discard the value. This can be used to use a map as a set.

The great use case for the unused variable is the situation when you only need a partial output. In the example below we only need to print the value (state population).

package main
import (
	"fmt"
)
func main() {
          statePopulations := map[string]int{
		  "California": 39250017,
		  "Texas":      27862596,
		  "Florida":    20612439,
	      }
	      for _, v := range statePopulations {
		  fmt.Println(v)
	}
}

It is called the blank identifier and it helps in cases where you wish to discard the value that is going to be returned and not reference it

Some places where we use it:

• A function returns a value and you don't intend to use it in the
  future
• You want to iterate and need an i value that you will not be
  using

Basically, _, known as the blank identifier. In GO we can't have variables that are not being used.

As an instance when you iterating through an array if you are using value := range you don't want a i value for iterating. But if you omit the i value it will return an error. But if you declare i and didn't use it, it will also return an error.

Therefore, that is the place where we have to use _,.

Also it is used when you don't want a function's return value in the future.

_(underscore) in Golang is known as the Blank Identifier. Identifiers are the user-defined name of the program components used for the identification purpose. Golang has a special feature to define and use the unused variable using Blank Identifier. Unused variables are those variables that are defined by the user throughout the program but he/she never makes use of these variables.
Go compiler throws an error whenever it encounters a variable declared but not used. Now we can simply use the blank identifier and not declare any variable at all.

   // Golang program to show the compiler
// throws an error if a variable is
// declared but not used

package main

import "fmt"

// Main function
func main() {

	// calling the function
	// function returns two values which are
	// assigned to mul and div identifier
	mul, div := mul_div(105, 7)

	// only using the mul variable
	// compiler will give an error
	fmt.Println("105 x 7 = ", mul)
}

// function returning two
// values of integer type
func mul_div(n1 int, n2 int) (int, int) {

	// returning the values
	return n1 * n2, n1 / n2
}

Output

./prog.go:15:7: div declared and not used

Let’s make use of the Blank identifier to correct the above program. In place of div identifier just use the _ (underscore). It allows the compiler to ignore declared and not used error for that particular variable.

    // Golang program to the use of Blank identifier

package main

import "fmt"

// Main function
func main() {

	// calling the function
	// function returns two values which are
	// assigned to mul and blank identifier
	mul, _ := mul_div(105, 7)

	// only using the mul variable
	fmt.Println("105 x 7 = ", mul)
}

// function returning two
// values of integer type
func mul_div(n1 int, n2 int) (int, int) {

	// returning the values
	return n1 * n2, n1 / n2
}

Output

105 x 7 =  735

> An unused variable is not allowed in Golang

If you were coming from other programming languages this might feel bit difficult to get used to this. But this results in more cleaner code. So by using a _ we are saying we know there is a variable there but we don't want to use it and telling the compiler that does not complain me about it.