base-4.20.2.0: Core data structures and operations

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
Safe Haskell | Trustworthy
Language | Haskell2010

Control.Monad.IO.Class

Description

Class of monads based on `IO`.

Synopsis

- class Monad m => MonadIO (m :: Type -> Type) where
  - liftIO :: IO a -> m a

# 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

MonadIO IO Source # | Since: base-4.9.0.0
Instance details Defined in Control.Monad.IO.Class Methods liftIO :: IO a -> IO a Source #