-- | The MonadTask class that defines the set of combinators to work with Task monad. -- -- The operations for MonadTask are similar to those of co-routines, with the -- addition of watching and signaling events. -- -- We also define a set of auto lifting for common transformers. Note that we -- purposely leave a case undefined where a state transformer goes on top of -- a task monad, because such an operation is either unsound or has to roll -- back the state (see @'Control.Monad.Trans.State.liftCallCC'@). So it's -- recommended to keep TaskT on top of all StateT in a transformer stack. {-# LANGUAGE UndecidableInstances, FunctionalDependencies, MultiParamTypeClasses, FlexibleInstances #-} module Control.Monad.Task.Class ( -- * MonadTask class MonadTask(..) , orElse ) where import Data.Monoid import Control.Monad.Cont import Control.Monad.Trans.Identity import Control.Monad.Trans.List import Control.Monad.Trans.Maybe import Control.Monad.Trans.Reader import qualified Control.Monad.Trans.Writer.Lazy as LazyWriter import qualified Control.Monad.Trans.Writer.Strict as StrictWriter import Control.Monad.Trans.Except -- | @MonadTask@ specifies a task monad @m@ over an event type @e@. class Monad m => MonadTask e m | m -> e where -- | @yield@ temporarily suspends current task to let others run. yield :: m () -- | @fork@ spawns a task and runs it immediately until it ends or -- suspends before returning to current task. fork :: m a -> m () -- | @watch@ suspends the current task to wait for future events, and will -- resume execution when an event triggers its watching function. watch :: (e -> Maybe a) -> m a -- | @signal@ broadcasts an event to all other tasks that are watching, -- and give those who wake up the priority to run. signal :: e -> m () -- | @exit@ ends all tasks and returns immediately. exit :: m () -- | @orElse@ is a helper function for combining two trigger functions -- disjuctively, favoring the first one. orElse :: (e -> Maybe a) -> (e -> Maybe b) -> e -> Maybe (Either a b) orElse f g x = maybe (fmap Right (g x)) (Just . Left) (f x) instance (Monad m, MonadTask a m) => MonadTask a (ExceptT e m) where exit = lift exit yield = lift yield fork = lift . fork . runExceptT watch = lift . watch signal = lift . signal instance (Monad m, MonadTask a m) => MonadTask a (IdentityT m) where exit = lift exit yield = lift yield fork = lift . fork . runIdentityT watch = lift . watch signal = lift . signal instance (Monad m, MonadTask a m) => MonadTask a (ListT m) where exit = lift exit yield = lift yield fork = lift . fork . runListT watch = lift . watch signal = lift . signal instance (Monad m, MonadTask a m) => MonadTask a (MaybeT m) where exit = lift exit yield = lift yield fork = lift . fork . runMaybeT watch = lift . watch signal = lift . signal instance (Monad m, MonadTask a m) => MonadTask a (ReaderT r m) where exit = lift exit yield = lift yield fork = ReaderT . (fork .) . runReaderT watch = lift . watch signal = lift . signal instance (Monoid w, Monad m, MonadTask a m) => MonadTask a (LazyWriter.WriterT w m) where exit = lift exit yield = lift yield fork = lift . fork . LazyWriter.runWriterT watch = lift . watch signal = lift . signal instance (Monoid w, Monad m, MonadTask a m) => MonadTask a (StrictWriter.WriterT w m) where exit = lift exit yield = lift yield fork = lift . fork . StrictWriter.runWriterT watch = lift . watch signal = lift . signal