Haskell Signal Handler

signalHandler.hs

#!/usr/bin/env nix-shell
#!nix-shell --pure -p "haskellPackages.ghcWithPackages (pkgs: [])" -i "runghc"

{-
Installing unix signal handler
-}
import           Control.Concurrent      (threadDelay)
import           Control.Concurrent.MVar (MVar, newEmptyMVar, putMVar,
                                          tryTakeMVar)
import           System.Posix.Signals    (Handler (CatchOnce), installHandler,
                                          sigABRT, sigKILL, sigQUIT, sigTERM, sigSTOP)

termHandler :: String -> MVar () -> Handler
termHandler signalName v = CatchOnce $ do
    putStrLn $ "Caught " <> signalName
    putMVar v ()

loop :: MVar () -> IO ()
loop v = do
    putStrLn "Still running"
    threadDelay 1000000
    val <- tryTakeMVar v
    case val of
        Just _  -> putStrLn "Quitting" >> pure ()
        Nothing -> loop v

main :: IO ()
main = do
    v <- newEmptyMVar
    installHandler sigTERM (termHandler "SIGTERM" v) Nothing
    installHandler sigSTOP (termHandler "SIGSTOP" v) Nothing
    installHandler sigQUIT (termHandler "SIGQUIT" v) Nothing
    installHandler sigABRT (termHandler "SIGABRT" v) Nothing
    -- | this signal is not catchable.
    -- See https://hackage.haskell.org/package/unix-2.8.1.1/docs/System-Posix-Signals.html#v:sigKILL
    installHandler sigKILL (termHandler "SIGKILL" v) Nothing
    loop v