I have a code that reads files and parses using UU.Parsing lib that returns an abstract sintax tree and shows on the screen. I received the error message "No instance for Show" in my functions originated intokensParserToByteString and applyParser using parseIO (of UU.Parsing lib) and inherited signatures until main. I fixed the signatures but my problem is in the main function. I added the instance Show in the signature but I have the next compilation error: No instance for (Show (IO J2s)) arising from a use of ‘main’ In the expression: main When checking the type of the IO action ‘main’ Some idea, about the problem? Main module ----------------------------------------------------------------------- {-# LANGUAGE FlexibleContexts #-} module Main where import UU.Parsing ... import Content main :: (Show (IO J2s)) => IO() main = do f <- getLine let command = test f command test :: (Show (IO J2s)) => String -> IO() test "testparser" = testParser Test module ----------------------------------------------------------------------- {-# LANGUAGE FlexibleContexts #-} module J2s.Parser.Test where import Content import J2s.Ast.Sintax import J2s.Parser import UU.Parsing ... testParser :: (Show (IO J2s)) => IO() testParser = (runSafeIO $ runProxy $ runEitherK $ contentsRecursive "path/of/my/tests" />/ handlerParser) :: (Show (IO J2s)) => IO() Content module ----------------------------------------------------------------------- {-# LANGUAGE FlexibleContexts #-} module Content where import Control.Monad(forM, liftM) import System.Directory (doesDirectoryExist, getDirectoryContents) import System.FilePath ((), splitExtension, splitFileName) import J2s.Parser import J2s.Ast.Sintax import UU.Parsing import Control.Monad (when, unless) import Control.Proxy import Control.Proxy.Safe hiding (readFileS) import J2s.Scanner.Token import Text.Show import UU.Parsing contentsRecursive :: (CheckP p) => FilePath -> () -> Producer (ExceptionP p) FilePath SafeIO () contentsRecursive path () = loop path where loop path = do contents path () //> \newPath -> do respond newPath isDir <- tryIO $ doesDirectoryExist newPath let isChild = not $ takeFileName newPath `elem` [".", ".."] when (isDir && isChild) $ loop newPath applyParser :: (Proxy p, Show (IO J2s)) => String -> Consumer p B.ByteString IO () applyParser path = runIdentityP loop where loop = do bs <- request () let sc = classify (initPos path) (B8.unpack bs) lift $ B8.putStrLn (tokensParserToByteString sc) tokensParserToByteString :: (Show (IO J2s)) => [Token] -> B.ByteString tokensParserToByteString tokens = B8.pack(show (parseIO pJ2s tokens)) handlerParser :: (CheckP p, Show (IO J2s)) => FilePath -> Session (ExceptionP p) SafeIO () handlerParser path = do canRead <- tryIO $ fmap readable $ getPermissions path isDir <- tryIO $ doesDirectoryExist path isValidExtension <- tryIO $ evaluate ((snd (splitExtension path) == ".java" || snd (splitExtension path) == ".mora") && (snd (splitFileName path) /= "EncodeTest.java") && (snd (splitFileName path) /= "T6302184.java") && (snd (splitFileName path) /= "Unmappable.java")) when (not isDir && canRead && isValidExtension) $ (readFileSP 10240 path >-> try . applyParser) path readFileSP :: (CheckP p) => Int -> FilePath -> () -> Producer (ExceptionP p) B.ByteString SafeIO () readFileSP chunkSize path () = bracket id (openFile path ReadMode) hClose $ \handle -> do let loop = do eof <- tryIO $ hIsEOF handle unless eof $ do bs <- tryIO $ B.hGetSome handle chunkSize respond bs loop loop -- Tatiana Andrea Moruno Rodriguez