Hi, I'm stuck with maintaining a State in a Haskeline program that would survive an interrupt with Ctrl-C. The following is a MWE I have distilled out of a bigger project. It revolves around a simple (I guess) read-eval-print loop using Haskeline, and a stacked StateT to maintain a state. This MWE reads a line from the user, calculates its `length`, and adds the length to an Int forming the state. If the user types the special input "sleep" the program sleeps for 5 seconds, simulating a longer computation. Every time the user presses Ctrl-C * a running computation should be interrupted, or * when at the prompt, the current input should be cleared. * In any case, the state must be maintained! Unfortunately, Ctrl-C also clears the state. At first I thought I had stacked the `StateT` and `InputT` in the wrong order. But changing from `InputT (StateT Int IO) ()` to `StateT Int (InputT IO) ()` seems not to change anything (which really gives me the creeps). This message contains both versions (one commented out), they compile with $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.10.1 $ ghc --make -Wall -outputdir tmp -o mwe StateMwe.lhs Example run, comments (`<--...`) added later: $ ./mwe type> arglschluargl Adding length of "arglschluargl" 13 type> lalala Adding length of "lalala" 19 type> <--no input, state unchanged Adding length of "" 19 type> sleep Sleeping 5 seconds... ^C <--Here I have hit C-c type> <--no input, state unchanged Adding length of "" 0 <--WRONG: that should be 19 Here we go:

import Control.Monad.State.Strict import Control.Concurrent ( threadDelay ) import System.Console.Haskeline

{-

Version one: `StateT` inside `InputT`

main :: IO () main = evalStateT (runInputT defaultSettings $ noesc repl) 0

This catches an interrupt via Ctrl-C and restarts the passed operation.

noesc :: MonadException m => InputT m a -> InputT m a noesc w = withInterrupt $ let loop = handle (\Interrupt -> loop) w in loop

The read-eval-print loop: EOF terminates, `sleep` delays, and any other input modifies the integer state.

repl :: InputT (StateT Int IO) () repl = do x <- getInputLine "\ntype> " case x of Nothing -> return () Just "sleep" -> do outputStrLn "Sleeping 5 seconds..." lift . lift . threadDelay $ 5 * 10^(6::Int) outputStrLn "...not interrupted" repl Just t -> do outputStrLn $ "Adding length of " ++ show t lift $ modify (+ length t) v <- lift get outputStrLn $ show v repl

-}

{-

Version two: `InputT` inside `StateT`

main :: IO () main = runInputT defaultSettings . noesc $ evalStateT repl 0

This catches an interrupt via Ctrl-C and restarts the passed operation. I suspect that I have to rearrange this to accommodate the modified stacking, but I could not come up with anything that compiles...

noesc :: MonadException m => InputT m a -> InputT m a noesc w = withInterrupt $ let loop = handle (\Interrupt -> loop) w in loop

As above, with `lift` in different places. To get a better understanding of what's going on, I do not want to use mtl's lift-to-the-right-monad magic (yet).

repl :: StateT Int (InputT IO) () repl = do x <- lift $ getInputLine "\ntype> " case x of Nothing -> return () Just "sleep" -> do lift $ outputStrLn "Sleeping 5 seconds..." lift . lift . threadDelay $ 5 * 10^(6::Int) lift $ outputStrLn "...not interrupted" repl Just t -> do lift . outputStrLn $ "Adding length of " ++ show t modify (+ length t) v <- get lift . outputStrLn $ show v repl

-}

Any help would be welcome... Stefan -- http://stefan-klinger.de o/X /\/ \