Description

This module defines the class MonadPeelIO of IO-based monads into which control operations on IO (such as exception catching; see Control.Exception.Peel) can be lifted.

liftIOOp and liftIOOp_ enable convenient lifting of two common special cases of control operation types.

Synopsis

# Documentation

MonadPeelIO is the class of IO-based monads supporting an extra operation peelIO, enabling control operations on IO to be lifted into the monad.

Minimal complete definition

peelIO

Methods

peelIO :: m (m a -> IO (m a)) Source #

peelIO is a version of peel that operates through an arbitrary stack of monad transformers directly to an inner IO (analagously to how liftIO is a version of lift). So it can be used with liftIO to lift control operations on IO into any monad in MonadPeelIO. For example:

foo :: IO a -> IO a
foo' :: MonadPeelIO m => m a -> m a
foo' a = do
k <- peelIO  -- k :: m a -> IO (m a)
join $liftIO$ foo (k a)  -- uses foo :: IO (m a) -> IO (m a)

Note that the "obvious" term of this type (peelIO = return return) does not work correctly. Instances of MonadPeelIO should be constructed via MonadTransPeel, using peelIO = liftPeel peelIO.

Instances

liftIOOp :: MonadPeelIO m => ((a -> IO (m b)) -> IO (m c)) -> (a -> m b) -> m c Source #

liftIOOp is a particular application of peelIO that allows lifting control operations of type (a -> IO b) -> IO b (e.g. alloca, withMVar v) to MonadPeelIO m => (a -> m b) -> m b.

liftIOOp f g = do
k <- peelIO
join $liftIO$ f (k . g)

liftIOOp_ :: MonadPeelIO m => (IO (m a) -> IO (m b)) -> m a -> m b Source #

liftIOOp_ is a particular application of peelIO that allows lifting control operations of type IO a -> IO a (e.g. block) to MonadPeelIO m => m a -> m a.

liftIOOp_ f m = do
k <- peelIO
join $liftIO$ f (k m)