Why haskell gnuplot code works in ghci, but not from CLI after compiling?

Question

The next simple code works fine in GHCi (the window with graphics appears), but after compiling in GHC, when one run it from command line, nothing happens. Why is it so?

import Graphics.Gnuplot.Simple

main :: IO ()
main = do
  plotFunc [] (linearScale 1000 (-10.0::Double,10.0)) (\x -> x^2)

GHC 8.2.2 gnuplot 0.5.5.1

Answer 1

Don't exit your program immediately after opening the window, since that will close it down.

For instance, wait for the user:

import Graphics.Gnuplot.Simple

main :: IO ()
main = do
  plotFunc [] (linearScale 1000 (-10.0::Double,10.0)) (\x -> x^2)
  putStrLn "Press enter to exit."
  getLine
  return ()

Answer 2

Your best bet is to switch from Graphics.Gnuplot.Simple to Graphics.Gnuplot.Advanced. You can use plotSync to wait for GNUPlot to exit. I don't think plotAsync has much to offer, since it doesn't offer a way to wait for the thread it creates. Here's a better version:

myPlotAsync
  :: (Terminal.C terminal, Display.C gfx)
  => terminal -> gfx -> IO (ThreadId, MVar ExitStatus)
myPlotAsync term gfx = do
  resultMV <- newEmptyMVar
  tid <- forkIO $ plotSync term gfx
             >>= putMVar resultMV
          `onException` putMVar resultMV ExitSuccess
  pure (tid, resultMV)

After your program calls myPlotAsync, it can do whatever else it needs to do and then call readMVar on the MVar to wait for GNUPlot to exit and get its exit status. It also has the option of holding on to the thread ID so it can kill the GNUPlot thread using throwTo.