Why is the go compiler flagging my variable as unused?

Question

my first question in StackOverflow :D

I am running on go 1.16. I created this function:

func (_m *MyPool) InChannel(outs ...chan interface{}) error {
    for _, out := range outs {
        out = _m.inChan
    }
    return nil
}

MyPool is a worker pool type, containing, among other members:

type MyPool struct {
    inChan          chan interface{}
}

My main problem is Go is flagging the out loop variable when ranging over the Variadic input of InChannel. Why is that? I am indeed using it...

Sorry, I am a noob at StackOverflow, so I am editing to clarify a bit. I indeed want to assign, not send. This is because the sender will have an outChan chan interface{} as a member variable, and will send values via:

func (s *Sender) Out(out interface{}) {
    select {
    case <-s.Ctx.Done():
        return
    case s.outChan <- out:
        return
    }
}

EDIT: So I ended up solving it by doing:

func (m *MyPool) InChannel(outs ...*chan interface{}) error {
    for _, out := range outs {
        *out = m.inChan
    }
    return nil
}

Answer 1

You're not "really" using it. You assign something to a variable which you don't read, so the assignment has no effect, so basically you're not using the variable.

Note that = is assignment. If you want to send something on the channel, use the send statement:

out <- _m.inChan

Or maybe you want to change the value out represents? out is a loop variable, it's a copy of the slice elements you range over. Assigning a value to out only assigns a value to the loop variable, but not to the slice element.

In general you may change slice elements by assigning a value using an index expression like this:

s := []int{1, 2, 3}
for i := range s {
    s[i] = 10 // Assign a value to the slice elements
}

In your case however this would do no good, since you're ranging over the slice of the variadic parameters.