prolude-0.0.0.30: ACI Learning's custom prelude
Safe HaskellSafe-Inferred
LanguageHaskell2010

Prolude.MonadIO

Synopsis

Documentation

class Monad m => MonadIO (m :: Type -> Type) 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:

Methods

liftIO :: IO a -> m a #

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

Expand
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

Instances

Instances details
MonadIO IO

Since: base-4.9.0.0

Instance details

Defined in Control.Monad.IO.Class

Methods

liftIO :: IO a -> IO a #

MonadIO Q 
Instance details

Defined in Language.Haskell.TH.Syntax

Methods

liftIO :: IO a -> Q a #

MonadIO m => MonadIO (LoggingT m) 
Instance details

Defined in Control.Monad.Logger

Methods

liftIO :: IO a -> LoggingT m a #

MonadIO m => MonadIO (NoLoggingT m) 
Instance details

Defined in Control.Monad.Logger

Methods

liftIO :: IO a -> NoLoggingT m a #

MonadIO m => MonadIO (WriterLoggingT m) 
Instance details

Defined in Control.Monad.Logger

Methods

liftIO :: IO a -> WriterLoggingT m a #

MonadIO m => MonadIO (ResourceT m) 
Instance details

Defined in Control.Monad.Trans.Resource.Internal

Methods

liftIO :: IO a -> ResourceT m a #

MonadIO m => MonadIO (ListT m) 
Instance details

Defined in Control.Monad.Trans.List

Methods

liftIO :: IO a -> ListT m a #

MonadIO m => MonadIO (MaybeT m) 
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

liftIO :: IO a -> MaybeT m a #

MonadIO m => MonadIO (RandT g m) 
Instance details

Defined in Control.Monad.Trans.Random.Lazy

Methods

liftIO :: IO a -> RandT g m a #

MonadIO m => MonadIO (RandT g m) 
Instance details

Defined in Control.Monad.Trans.Random.Strict

Methods

liftIO :: IO a -> RandT g m a #

(Functor f, MonadIO m) => MonadIO (FreeT f m) 
Instance details

Defined in Control.Monad.Trans.Free

Methods

liftIO :: IO a -> FreeT f m a #

(Monoid w, Functor m, MonadIO m) => MonadIO (AccumT w m) 
Instance details

Defined in Control.Monad.Trans.Accum

Methods

liftIO :: IO a -> AccumT w m a #

(Error e, MonadIO m) => MonadIO (ErrorT e m) 
Instance details

Defined in Control.Monad.Trans.Error

Methods

liftIO :: IO a -> ErrorT e m a #

MonadIO m => MonadIO (ExceptT e m) 
Instance details

Defined in Control.Monad.Trans.Except

Methods

liftIO :: IO a -> ExceptT e m a #

MonadIO m => MonadIO (IdentityT m) 
Instance details

Defined in Control.Monad.Trans.Identity

Methods

liftIO :: IO a -> IdentityT m a #

MonadIO m => MonadIO (ReaderT r m) 
Instance details

Defined in Control.Monad.Trans.Reader

Methods

liftIO :: IO a -> ReaderT r m a #

MonadIO m => MonadIO (SelectT r m) 
Instance details

Defined in Control.Monad.Trans.Select

Methods

liftIO :: IO a -> SelectT r m a #

MonadIO m => MonadIO (StateT s m) 
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

liftIO :: IO a -> StateT s m a #

MonadIO m => MonadIO (StateT s m) 
Instance details

Defined in Control.Monad.Trans.State.Strict

Methods

liftIO :: IO a -> StateT s m a #

(Monoid w, MonadIO m) => MonadIO (WriterT w m) 
Instance details

Defined in Control.Monad.Trans.Writer.Lazy

Methods

liftIO :: IO a -> WriterT w m a #

(Monoid w, MonadIO m) => MonadIO (WriterT w m) 
Instance details

Defined in Control.Monad.Trans.Writer.Strict

Methods

liftIO :: IO a -> WriterT w m a #

MonadIO m => MonadIO (ConduitT i o m) 
Instance details

Defined in Data.Conduit.Internal.Conduit

Methods

liftIO :: IO a -> ConduitT i o m a #

MonadIO m => MonadIO (ContT r m) 
Instance details

Defined in Control.Monad.Trans.Cont

Methods

liftIO :: IO a -> ContT r m a #

(Monoid w, MonadIO m) => MonadIO (RWST r w s m) 
Instance details

Defined in Control.Monad.Trans.RWS.Lazy

Methods

liftIO :: IO a -> RWST r w s m a #

(Monoid w, MonadIO m) => MonadIO (RWST r w s m) 
Instance details

Defined in Control.Monad.Trans.RWS.Strict

Methods

liftIO :: IO a -> RWST r w s m a #

MonadIO m => MonadIO (Pipe l i o u m) 
Instance details

Defined in Data.Conduit.Internal.Pipe

Methods

liftIO :: IO a -> Pipe l i o u m a #