go: `:=` operator causes accidental shadowing

January 14, 2016

Posted in go

Go provides := operator to make declaring variables easier. It is a shorthand to declare and set a value of a variable. for example,

var x int
x = 42

can be written as

x := 42

But if not careful, this can accidently shadow variable bindings. Let’s look at the fictitious piece of code.

package main

import "fmt"


func fictitiousFunc() (int, error) {
	return 42, nil
}

func main() {
	x := 10;
	x, err := fictitiousFunc()
    if err != nil {
		fmt.Println("I'll never print")
    }
	fmt.Println("value of x: ", x)
}

This produces following output

value of x:  42

While, this following piece of code will fail to compile

package main

import "fmt"

func fictitiousFunc() (int, error) {
    return 42, nil
}

func main() {
    x := 10
    // replace :=
    var x int
    var err error
    x, err = fictitiousFunc()
    if err != nil {
        fmt.Println("I'll never print") 
    }
    fmt.Println("value of x: ", x)
}

output:

prog.go:12: x redeclared in this block
    previous declaration at prog.go:10

So we can see that the operator is somewhat intelligent, and does not redeclare the variables.

Now what if we push it down a scope? See the following code

package main

import "fmt"


func fictitiousFunc() (int, error) {
	return 42, nil
}

func main() {
	someCondition := true
	
	x := -1;
	
	if someCondition {
		x, err := fictitiousFunc()
		
		if err != nil {
			fmt.Println("I'll never print")
		}
		
		fmt.Println("value of x inside: ", x)
	}
	
	fmt.Println("value x outside: ", x)
	
}

This produces,

value of x inside:  42
value x outside:  -1

At line: 16, since the immediate scope (line:15-32) does not have variable x declared, := is redeclaring the variable. a.k.a the variable x gets shadowed.

Only workaround I can think of is not to use :=, i.e change the code to

	if someCondition {
        var err error
        x, err = fictitiousFunc()
		
		if err != nil {
			fmt.Println("I'll never print")
		}
		
		fmt.Println("value of x inside: ", x)
	}

If you know something better let me know.