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{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE RankNTypes #-}
{-|
Module : Utils.Monad
Copyright : (c) UU, 2019
License : MIT
Maintainer : Tom Smeding
Stability : experimental
Portability : POSIX, macOS, Windows
Some useful functions lifted to monadic actions.
-}
module Utils.Monad where
import Control.Monad.IO.Unlift
import Control.Monad.State.Strict (StateT (..), lift, get, put, MonadState, MonadTrans)
import Data.IORef
import Control.Exception (bracket, bracket_)
-- | If the boolean resolves to True, returns the first argument, else the
-- second.
ifM :: Monad m => m Bool -> m a -> m a -> m a
ifM b t e = b >>= \b' -> if b' then t else e
-- | Runs the monadic action only if the boolean resolves to True.
whenM :: Monad m => m Bool -> m () -> m ()
whenM b t = ifM b t (return ())
-- | whenJust m a = when (isJust m) (a (fromJust m)), but with less
-- fromJust.
whenJust :: Monad m => Maybe a -> (a -> m ()) -> m ()
whenJust = flip (maybe (return ()))
-- | 'bracket', but unlifted.
unliftBracket :: MonadUnliftIO m => m a -> (a -> m b) -> (a -> m b) -> m b
unliftBracket f g h =
withRunInIO $ \run ->
bracket (run f) (run . g) (run . h)
-- | 'bracket_', but unlifted.
unliftBracket_ :: MonadUnliftIO m => m a -> m b -> m c -> m c
unliftBracket_ f g h =
withRunInIO $ \run ->
bracket_ (run f) (run g) (run h)
newtype StateTUnliftIOWrapper s m a = StateTUnliftIOWrapper { runStateTUnliftIOWrapper :: StateT s m a }
deriving newtype (Functor, Applicative, Monad, MonadIO, MonadState s, MonadTrans)
instance MonadUnliftIO m => MonadUnliftIO (StateTUnliftIOWrapper s m) where
withRunInIO act = StateTUnliftIOWrapper $ do
state0 <- get
ref <- liftIO $ newIORef state0
let runner mrun (StateT mf) = do
s1 <- readIORef ref
(res, s2) <- runStateT (StateT $ mrun . mf) s1
writeIORef ref s2
return res
res <- lift $ withRunInIO $ \mrun -> act (runner mrun . runStateTUnliftIOWrapper)
liftIO (readIORef ref) >>= put
return res
|
