{-# LANGUAGE Safe #-}------------------------------------------------------------------------------- |-- Module : Control.Monad.IO.Class-- Copyright : (c) Andy Gill 2001,-- (c) Oregon Graduate Institute of Science and Technology, 2001-- License : BSD-style (see the file LICENSE)---- Maintainer : R.Paterson@city.ac.uk-- Stability : stable-- Portability : portable---- Class of monads based on @IO@.-----------------------------------------------------------------------------moduleControl.Monad.IO.Class(MonadIO (..))where-- | Monads in which 'IO' computations may be embedded.-- Any monad built by applying a sequence of monad transformers to the-- 'IO' monad will be an instance of this class.---- Instances should satisfy the following laws, which state that 'liftIO'-- is a transformer of monads:---- * @'liftIO' . 'return' = 'return'@---- * @'liftIO' (m >>= f) = 'liftIO' m >>= ('liftIO' . f)@class(Monad m )=>MonadIO m where-- | Lift a computation from the 'IO' monad.-- This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations-- (i.e. 'IO' is the base monad for the stack).---- === __Example__------ > import Control.Monad.Trans.State -- from the "transformers" library-- >-- > printState :: Show s => StateT s IO ()-- > printState = do-- > state <- get-- > liftIO $ print state------ Had we omitted @'liftIO'@, we would have ended up with this error:---- > • Couldn't match type ‘IO’ with ‘StateT s IO’-- > Expected type: StateT s IO ()-- > Actual type: IO ()---- The important part here is the mismatch between @StateT s IO ()@ and @'IO' ()@.---- Luckily, we know of a function that takes an @'IO' a@ and returns an @(m a)@: @'liftIO'@,-- enabling us to run the program and see the expected results:---- @-- > evalStateT printState "hello"-- "hello"---- > evalStateT printState 3-- 3-- @--liftIO ::IO a ->m a -- | @since 4.9.0.0instanceMonadIO IO whereliftIO :: forall a. IO a -> IO a
liftIO =IO a -> IO a
forall a. a -> a
id