How to Detect Unused Variables After Reassignment?

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1

Hello everyone,

I’ve been working with Go and encountered a situation where I need to detect unused variables that have been reassigned. Specifically, I have code where an error variable (err) is reassigned and not checked after the reassignment. Here’s a simplified example:

package main

import (
	"fmt"
	"strconv"
)

func main() {
	num1, err := strconv.Atoi("123")
	if err != nil {
		fmt.Printf("Error converting '123': %v\n", err)
		return
	}

	num2, err := strconv.Atoi("a")

	fmt.Println("Converted numbers:", num1, num2)
}
2

In any case you would need to add a check (if) for this variable. For example add this lines to your code, (1) before the secod Atoi and after that Atoi.

fmt.Printf("%v:%v\n", &err, err)

In the first case as any error happened, err is nil. and in the second print your err has some value.:

So it is better to check any error inmediately using something like :slight_smile:

if num2, err := strconv.Atoi("a"); err != nil {
   ...
}

That way you are sure it is a different variable from the previous one and it is only used in that scope, the narrow of teh scope, the better…

3

There are static code checkers which detect such, for example golangci-lint and staticcheck

$ golangci-lint run
x.go:15:8: ineffectual assignment to err (ineffassign)
	num2, err := strconv.Atoi("a")
$
$ staticcheck ./...
x.go:15:2: this value of err is never used (SA4006)
4

@Helmut_Wais

Thank you! I think I’ll adopt it since it can be detected by golangci-lint.