Here is what I have tried:

module Main where

import System.Process -- using runInteractiveProcess
import System.Posix.Types
import System.Process.Internals
import Control.Concurrent.MVar

main = do
(input,output,err,ph) <- runInteractiveProcess calc.exe" ([]) Nothing Nothing
p <- Main.getPID ph
putStrLn $ show p --- The output

getPID :: ProcessHandle -> IO CPid
getPID (ProcessHandle p) = do
(OpenHandle pp) <- takeMVar p
return (toPID pp)

toPID :: PHANDLE -> CPid
toPID ph = toEnum $ fromEnum ph

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But the CPid always returns some const value (like 1904 in cygwin; 1916 in cmd.exe)

I think, I saw that CPid was a newtype of Ptr ()

So, currently I need to get the process id from CPid or something else.

All help is appreciated.

John