base-4.20.0.0: Core data structures and operations
Copyright (c) Andy Gill 2001(c) Oregon Graduate Institute of Science and Technology 2001 BSD-style (see the file LICENSE) R.Paterson@city.ac.uk stable portable Trustworthy Haskell2010

Description

Class of monads based on IO.

Synopsis

# Documentation

class Monad m => MonadIO (m :: Type -> Type) where Source #

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)

Methods

liftIO :: IO a -> m a Source #

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
 Source # Since: base-4.9.0.0 Instance detailsDefined in Control.Monad.IO.Class MethodsliftIO :: IO a -> IO a Source #