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|
{-# 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 : experimental
-- 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
|
