Control.Monad.IO.Class.Compat

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

  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