How to send text to GHCi process?

Question

I'm working on Haskell presentation engine Howerpoint. It is running in GHCi. I would like to create a function which would output a statement to current running GHCi session. It must work on Linux and Mac, Windows is not necessary. Function probably will have type

executeStatement :: String -> IO ()

What I tried already:

• getProcessID and getParentProcessID and then sending something like

  echo 'xxx' > /proc/92856/fd/1
  -bash: /proc/92856/fd/1: No such file or directory
  

• I also tried runCommand but it executes command in the Bash and not in GHCi so I got error that the command was not found

• xdotool does not run on mac

Answer 1

You could use the ghcid project from hackage to evaluate expressions. They would not be evaluated in the same session as your are currently running, but you can send expressions and read their output in a session nonetheless. Here is an example:

import Language.Haskell.Ghcid

main :: IO ()
main = do 
    (g, _) <- startGhci "ghci" (Just ".") True 
    let executeStatement = exec g
    executeStatement "let x = 33"
    executeStatement "x + 8" >>= print . head
    executeStatement "print x" >>= print . head
    stopGhci g

The output is "41" "33" and g represents a ghci session.

If you really need to execute expressions in an already running ghci instance you can have a look at this function - startGhci and instead of creating a new process you would have to tap into the existing process and then set std_in, std_out and std_err.