String vs Char mismatch in haskell - haskell

Question

I'm getting an error about a type mismatch:
Main.hs:47:28:
Couldn't match type ‘[Char]’ with ‘Char’
Expected type: IO Char
Actual type: IO String
In the first argument of ‘liftIO’, namely ‘prompt’
In the second argument of ‘($)’, namely ‘liftIO prompt’
and struggling to understand why an IO Char is expected. Since prompt does type-check as IO String in line 46, and I thought, perhaps mistakenly, that liftIO would turn it to a String as suggested in this answer.
module Main where
import Syntax
import Parser
import Eval
import Pretty
import Counter
import Control.Monad
import Control.Monad.Trans
import System.Console.Haskeline
import Control.Monad.State
showStep :: (Int, Expr) -> IO ()
showStep (d, x) = putStrLn ((replicate d ' ') ++ "=> " ++ ppexpr x)
process :: Counter -> String -> InputT (StateT [String] IO) ()
process c line =
if ((length line) > 0)
then
if (head line) /= '%'
then do
modify (++ [line])
let res = parseExpr line
case res of
Left err -> outputStrLn $ show err
Right ex -> do
let (out, ~steps) = runEval ex
--mapM_ showStep steps
out_ps1 c $ out2iout $ show out
else do
let iout = handle_cmd line
out_ps1 c iout
-- TODO: don't increment counter for empty lines
else do
outputStrLn ""
out2iout :: String -> IO String
out2iout s = return s
out_ps1 :: Counter -> IO String -> InputT (StateT [String] IO) ()
out_ps1 c iout = do
--out <- liftIO iout
let out_count = c 0
let prompt = (getPrompt out_count iout) :: IO String
outputStrLn $ liftIO prompt
outputStrLn ""
getPrompt :: IO Int -> IO String -> IO String
getPrompt ion iout = do
n <- ion
out <- iout
return $ "Out[" ++ (show n) ++ "]: " ++ out
handle_cmd :: String -> IO String
handle_cmd line = if line == "%hist"
then
evalStateT getHist []
else
return "unknown cmd"
joinHist :: IO [String] -> IO String
joinHist ixs = do
xs <- ixs
return $ unlines xs
getHist :: StateT [String] IO String
getHist = do
hist <- lift get
let hists = (zip [(1::Int)..] hist) :: [(Int, String)]
return $ combineHist hists
combineHist :: [(Int, String)] -> String
combineHist hists = unlines $ map (\(i, h) -> show i ++ ": " ++ show h) hists
main :: IO ()
main = do
c <- makeCounter
repl c
repl :: Counter -> IO ()
repl c = evalStateT (runInputT defaultSettings(loop c)) []
loop :: Counter -> InputT (StateT [String] IO) ()
loop c = do
minput <- getLineIO $ in_ps1 $ c
case minput of
Nothing -> return ()
Just input -> process c input >> loop c
getLineIO :: (MonadException m) => IO String -> InputT m (Maybe String)
getLineIO ios = do
s <- liftIO ios
getInputLine s
in_ps1 :: Counter -> IO String
in_ps1 c = do
let ion = c 1
n <- ion
let s = "Untyped: In[" ++ (show n) ++ "]> "
return s
More context can be found here.

Answer 1

You are passing an IO action in place of a String to outputStrLn. You should instead do:
prompt <- liftIO $ getPrompt out_count iout
outputStrLn prompt
to obtain the String from the IO action, using liftIO and then passing this to outputStrLn.