lift package:streamly

Lift a computation from the argument monad to the constructed monad.

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

Lift the inner monad m of a stream t m a to tr m using the monad transformer tr.

A value associated with an IO action that is automatically called whenever the value is garbage collected.