可以选装接收器吗？

Is it possible to define an optional receiver for a function?

func (r ReceiverType) doSth(arg ArgType) (ReturnType) {
    if r == nil {
        return doSthWithoutReceiver()
    }

    return r.doSthWithReceiver()
}

Such that I can call the function as follows:

var sth ReceiverType
resWithoutRecv := doSth()
resWithRecv := sth.doSth()

Without the need of basically writing the same function twice, once with and once without the receiver? Since I didn't find anything I assume this is bad practice either way?

It's not possible to make the receiver argument optional, but you can declare a method and a function with the same name:

func (r ReceiverType) doSth(arg ArgType) (ReturnType) {
    return r.doSthWithReceiver()
}

func doSth(arg ArgType) (ReturnType) {
    return doSthWithoutReceiver()
}

and call them as stated in the question:

var sth ReceiverType
resWithoutRecv := doSth()
resWithRecv := sth.doSth()

You can also check for a nil receiver:

func (r *ReceiverType) doSth(arg ArgType) (ReturnType) {
    if r == nil {
        return doSthWithoutReceiver()
    }
    return r.doSthWithReceiver()
}

and call it like this:

var sth *ReceiverType
resWithoutRecv := sth.doSth()  // sth == nil