I'm getting a result from getInputline, whose type is:
(MonadException m) => IO String -> InputT m (Maybe String)
I'd like to get just the Maybe String part. I'm well aware that in general there is no way to strip a monad, as explained in this answer (and other answers in the same question). However, since I'm doing it inside an InputT, I guess it's possible, as proposed here. However, I can't just use liftIO, as the answer suggests, since the IO is inside a StateT.
loop :: Counter -> InputT (StateT [String] IO) ()
loop c = do
minput <- getLineIO $ in_ps1 $ c
case minput of
Nothing -> outputStrLn "Goodbye."
Just input -> (process' c input) >> loop c
getLineIO :: (MonadException m) => IO String -> InputT m (Maybe String)
getLineIO ios = do
s <- liftIO ios
getInputLine s
process' :: Counter -> String -> InputT (StateT [String] IO) ()
[...]
The error I'm getting:
Main.hs:81:15:
No instance for (MonadException (StateT [String] IO))
arising from a use of ‘getLineIO’
In the expression: getLineIO
In a stmt of a 'do' block: minput <- getLineIO $ in_ps1 $ c
In the expression:
do { minput <- getLineIO $ in_ps1 $ c;
case minput of {
Nothing -> outputStrLn "Goodbye."
Just input -> (process' c input) >> loop c } }
If I remove getLineIO and use getInputLine directly, as per @chepner's advice:
loop :: Counter -> InputT (StateT [String] IO) ()
loop c = do
minput <- (in_ps1 c) >>= getInputLine
case minput of
Nothing -> outputStrLn "Goodbye."
Just input -> (process' c input) >> loop c
I'm getting an error:
Main.hs:81:16:
Couldn't match type ‘IO’ with ‘InputT (StateT [String] IO)’
Expected type: InputT (StateT [String] IO) String
Actual type: IO String
In the first argument of ‘(>>=)’, namely ‘(in_ps1 c)’
In a stmt of a 'do' block: minput <- (in_ps1 c) >>= getInputLine
Full code can be found here, and explanations about what I'm trying to do can be found here.
