匹配类型和单子。 错误

Question

type C = StateT Int (ErrorT String IO)


main :: C ()
main = do
  args <- liftIO $ getArgs
  case args of
    [] -> liftIO $ putStrLn "E"
    (p:ps) -> s "d"

s :: String -> C ()

I've got the error:

     Couldn't match type ‘Control.Monad.Trans.State.Lazy.StateT
                           Int
                           (Control.Monad.Trans.Error.ErrorT String IO)
                           ()’
                   with ‘IO t0’
    Expected type: IO t0
      Actual type: C ()
    In the expression: main
    When checking the type of the IO action ‘main’

I cannot understand why there is an error and how to repair it. Please help.

Answer 1

main must be of type IO a for some a (usually, but not necessarily, () ). It can't be of type StateT Int (ErrorT String IO) . You can fix this by using the functions provided with StateT and ErrorT for converting them to actions of the underlying monad.