Vim ch_sendexpr(), Send data from Server to Vim client

        
        let g:tcpChannel = ch_open('localhost:4242', {'callback': 'HaskellServer_Handler', 'mode':'json'})

        ↓  

        call ch_sendexpr(g:tcpChannel, 'hello', {'callback' : 'HaskellServer_Handler'})
        ↓  

        " TCP server side will receive the following
        [1, "Hello"]
         ↑ 
        msg id

        " Send data from Server to Vim client from Haskell TCP Server.
        " NOTE: Double quotes are IMPORTANT here
        
                   Need it in Haskell code
                   ↓      ↓ 
        send "[1, \"reply\"]"  ✅ 
               ↑  
              msg id

        msg id needs to match msg id from Vim client

        Or msg id can be zero
        send "[0, reply]"        ❌ 
        send "[0, \"reply\"]"    ✅ 
               ↑ 
              msg id


        vim client               TCP server
        + - - - +                + - - - +
        |       |  →   'hello'   |       | [1, "hello"]
        + - - - +                + - - - + 
        |       |  →   'world'   |       | [2, "world"]
        + - - - +                + - - - +
        |       |  →   'Bye'     |       | [3, "Bye"]
        + - - - +                + - - - +

         msg id [1, 2, 3] <-> msg id [1, 2, 3]
                    ↓                       ↓  
        vim client               TCP server
        + - - - +                + - - - +
        |       | ← [1 'hello']  |       | [1, "hello"]
        + - - - +                + - - - + 
        |       | ← [2, 'world'] |       | [2, "world"]
        + - - - +                + - - - +
        |       | ← [3, 'Bye']   |       | [3, "Bye"]
        + - - - +                + - - - +
                
                msg id = 0
                     ↓  
        vim client               TCP server
        + - - - +                + - - - +
        |       | ← [0 'anyab']  |       | [1, "hello"]
        + - - - +                + - - - + 
        |       | ← [0, 'what '] |       | [2, "world"]
        + - - - +                + - - - +
        |       | ← [0, 'bye']   |       | [3, "Bye"]
        + - - - +                + - - - +

        SEE: $sp/haskell-vim-channel  😀   

    func! HaskellServer_Init()
        if exists('g:tcpChannel')
            if ch_status(g:tcpChannel) == 'open'
                call ch_close(g:tcpChannel)
            endif
        endif
        sleep 1 
         let g:tcpChannel = ch_open('localhost:4242', {'callback': 'HaskellServer_Handler', 'mode':'json'})

        echo ch_status(g:tcpChannel)
        call ch_logfile('/tmp/2.x', 'w')
    endfunc

    func! HaskellServer_Send(mydata)
        call ch_sendexpr(g:tcpChannel, a:mydata, {'callback' : 'HaskellServer_Handler'})
    endfunc

    func! HaskellServer_Handler(tcpChannel, msg)
       echo "From handler toUpperStr => " . a:msg
       call writefile([a:msg], '/tmp/aaa.x', 'a')
    endfunc

    " Send a list of string to the channel 
    func! HaskellServer_SendVisualLine()
        " let ls = VisualSelectLine()
        let s = @" 
        let xs = split(s, "\n")
        let xt = ["beg"] + xs + ["end"]
        call writefile([s], '/tmp/a.x', 'b')
        let time = reltime()[0]
        let tstr = 'time=' . time
        for ux in xt 
            call HaskellServer_Send(ux)
        endfor
    endfunc