{-# 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@. ----------------------------------------------------------------------------- module Control.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 4.9.0.0 instance MonadIO IO where liftIO = id