I'm confused with unknown for bindings

Greetings.

I read the Unknown for type safety section, but then I got stuck. How to actually make getPropertySafe work?

I was writing bindings for an external library. There is a sub/push mechanism. I think you guys can got an idea from the following code:

websocket.onPush = (cmd, res)=>{
        if(ftCmdID.QotUpdateOrderBook.cmd == cmd){
            let { retType, s2c } = res
            if(retType == RetType.RetType_Succeed){
                console.log("OrderBookTest", s2c);
            } else {
                console.log("OrderBookTest: error")
            }
        }
    };

I wrote the binding for onPush like this: @send external onpush: (ftwebsocket, (~cmd: int, ~res: unknown) => unit) => unit = "onPush". The res type shape depends on the matched cmd.

What’s the proper way to do these kind of bindings? Thanks!