Why Does My Go Code Report a \'Declared and Not Used\' Error on the `prev` Variable Despite Its Use?

Declared and Unused Error in Go, Despite Variable Usage

In the following Go code, the compiler reports a "declared and not used" error on the variable prev.

<code class="go">package main

import "fmt"

// fibonacci is a function that returns
// a function that returns an int.
func fibonacci() func() int {
    prev := 0
    curr := 1
    return func() int {
        temp := curr
        curr := curr + prev
        prev := temp
        return curr
    }
}

func main() {
    f := fibonacci()
    for i := 0; i < 10; i++ {
        fmt.Println(f())
    }
}</code>

Understanding the Error

The compiler correctly identifies that the variable prev is declared but not used. This means that the declared prev variable in the function fibonacci is never referenced in the code.

Resolving the Issue

The error can be resolved by modifying the fibonacci function to correctly use the prev variable. The intent of the code is likely to modify the prev variable with the prev := temp assignment. However, this assignment creates a new local variable named prev that is hidden from the surrounding scope. Instead, the correct assignment should use the = operator without the declaration keyword:

<code class="go">func fibonacci() func() int {
    prev := 0
    curr := 1
    return func() int {
        temp := curr
        curr := curr + prev
        prev = temp
        return curr
    }
}</code>

By removing the declaration keyword (:=) from the second prev assignment, the code correctly modifies the inherited prev variable. This resolves the compiler error and allows the code to function as intended.

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