Why Does Go's Compiler Report 'Declared But Not Used' for an Error Variable Used in a Loop?

Compiler Confusion in Go: Understanding Declared But Unused Variables

In Go, the compiler plays a crucial role in ensuring code correctness. Sometimes, however, it may flag errors that seem puzzling at first glance, such as "variable declared but not used." Let's delve into a specific example to uncover the root cause of this issue.

Example: Declared but Unused Error Variable in IO Operations

Consider the following code snippet that utilizes the io package:

func main() {
    readers := []io.Reader{
         strings.NewReader("from string reader"),
         bytes.NewBufferString("from bytes reader"),
    }

    reader := io.MultiReader(readers...)
    data := make([]byte, 1024)

    var err error
    //var n int

    for err != io.EOF {
        n, err := reader.Read(data)
        fmt.Printf("%s\n", data[:n])
    }
    os.Exit(0)
}

The compiler flags an error: "err declared and not used." However, the code uses the err variable in the for statement, so this error seems counterintuitive.

Shadowing and Variable Declarations in Go

The key to understanding the issue lies in the use of := within the for loop. This syntax declares a new err variable within the loop scope, which shadows the original err declared outside the loop. As a result, the compiler no longer tracks the original err variable, hence the "declared but not used" error.

Remedying the Issue

To resolve the issue, one can explicitly mention both err variables within the loop scope:

for var err error; err != io.EOF; {
        n, err := reader.Read(data)
        fmt.Printf("%s\n", data[:n])
    }

Alternatively, one can avoid the shadowing issue by using a different variable name for the loop scope, such as loopErr:

for loopErr := err; loopErr != io.EOF; {
        n, loopErr := reader.Read(data)
        fmt.Printf("%s\n", data[:n])
    }

By addressing the shadowing issue, the compiler error is eliminated, and the code functions as intended.

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