Why Does My Go Code Show 'err declared but not used' Despite Using `err` in a Loop?

Shadowing Variables in Go: Understanding the "err declared but not used" Error

When programming in Go, it's essential to understand the concept of variable shadowing to avoid common compiler errors. This article explores a specific instance where the compiler flags a variable as declared but not used due to shadowing.

In the provided code example, a common error encountered by novice Go programmers is: "err declared and not used." This error refers to the err variable inside the for loop. Upon closer examination, it appears that err is used within the loop's condition. So, why does the compiler raise this error?

The issue arises because the err declared within the for loop shadows the err variable declared outside the loop. Shadowing occurs when a variable is redeclared with the short variable declaration (:=), assigning it local scope. In this case, the err variable inside the loop is initialized as a new variable, hiding the one declared outside the loop.

Consequently, the err variable used in the for loop's condition refers to the shadowed variable with local scope, while the outer loop's err remains unused. This leads the compiler to flag err as declared but not utilized in the outer loop.

To resolve this error, one should remove the shadowing by declaring err outside the for loop or using a different variable name for the inner scope, ensuring that the outer loop's err variable remains referenced.

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