为什么编译器会在此实例中抱怨一个未使用的变量（当fmt.Fprintf使用它时）？

Question

I have a simple piece of code where I want to convert elements of a slice into json and then print them out to my http.responseWriter.

for _, element := range customers {
    result, _ := json.Marshal(element)
    fmt.Fprintf(w, string(result))
}

However when I compile this I get the error "result declared and not used". If I add a simple line:

_ = result

Then everything compiles and works fine. Why does the compiler complain about this usage, and what is the correct way to do this in go?

Any insight is appreciated, my searches so far seem to indicate the call to Fprintf should count as a usage.

Answer 1

The code in question does not result in the error posted, for proof, check it on the Go Playground.

This error usually is (and the op confirmed it is too in this case) caused by having a local variable with same name outside of the block, and when using the short variable declaration, that shadows that variable.

This error can be reproduced with the following code:

var result []byte

customers := []int{}
w := os.Stdout

for _, element := range customers {
    result, _ := json.Marshal(element)
    fmt.Fprintf(w, string(result))
}

Attempting to compile and run it, we get the error (try it on the Go Playground):

prog.go:10:6: result declared and not used

Solution is to use a simple assignment instead of the short variable declaration if intention is to use the existing variable (in which case no new variable will be created), or use a different name for the variable if intention is not to use the outer, existing variable (but then the outer variable is to be removed or be used of course).