Why Does My Go Global Error Variable Remain Nil After Initialization?

Global Error Variable Remains Nil After Initialization

When attempting to initialize an error variable globally, some users may encounter a puzzling issue where the variable remains nil in subsequent functions within the same package, leading to unexpected program behavior. This issue stems from the way global variables are initialized in Go.

In the following example, the goal is to initialize a global error variable loadErr and check for errors in another function checkErr():

package main

import (
    "os"
    "fmt"
)

var loadErr error

func main() {
    f, loadErr := os.Open("asdasd")
    if loadErr != nil {
        checkErr()
    }
    if f != nil {
        fmt.Println(f.Name())
    }
}

// panic won't be called because loadErr is nil
func checkErr() {
    if loadErr != nil {
        panic(loadErr)
    }
}

However, the issue arises because checkErr() receives a nil value for loadErr. This is because the variable created in main() ("loadErr := os.Open('asdasd')") is a local variable within main(), and the global variable is not initialized at this point.

To resolve this issue, one should use a simple assignment ("=") instead of a declaration and initialization (":=") when assigning the value to the global variable:

// ...

func main() {
    _, loadErr = os.Open("asdasd")
    // ^ Replaced := with =
    if loadErr != nil {
        checkErr()
    }
    // ...
}

By using the "=" operator, the global variable loadErr` is explicitly assigned the error value, ensuring that it is accessible to other functions within the package.

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