Control.Monad.IOT

Documentation

data IOT m t

An IO monad transformer.

IOT cannot be unwrapped in the usual way -- the monad inside it has to be unwrapped. This is done using run, and hoist from mmorph.

Most of the safety of the IO monad is ensured statically. However, to ensure that the same RealWorld token is not used multiple times, a runtime check is necessary. Among the alternatives that perform I/O, the first alternative forced by a concatenation of hoists will contain a result, and subsequent alternatives will be errors.

Therefore, a concatenation of hoists out of a monad defines at most one path of RealWorld token use. Here is an example using the binary tree monad:

>>> let io :: IOT Tree () = lift (Node (Leaf 1) (Leaf 2)) >>= liftIO . print

>>> run $ hoist (\(Node (Leaf x) _) -> Identity x) io
1

>>> run $ hoist (\(Node _ (Leaf x)) -> Identity x) io
2

>>> run $ hoist (\(Node (Leaf _) (Leaf x)) -> Identity x) io
1
*** Exception: IOT: double RealWorld use

Instances

MFunctor IOT

MMonad IOT

MonadTrans IOT

Monad m => Monad (IOT m)

Monad m => Functor (IOT m)

Monad m => Applicative (IOT m)

Monad m => MonadIO (IOT m)

run :: IOT Identity t -> IO t

Run an IOT yielding an IO computation. The Identity monad is a trivial wrapper around IO.

module Control.Monad.Trans

module Control.Monad.Identity

module Control.Monad.Morph