Add monad effects

src/Control/Ev/Monad.hs

@@ -0,0 +1,87 @@
+{-# LANGUAGE RankNTypes, FlexibleContexts, TypeOperators, UndecidableInstances #-}
+{-|
+Description : Definitions for monad effects.
+Copyright   : (c) 2021 Jaro Reinders
+License     : MIT
+Maintainer  : [email protected]; [email protected]
+Stability   : Experimental
+
+Some definitions for monad effects.
+
+The effect monad can include at most one other monad effect.
+A common effect to include is the IO effect, which allows you to run IO actions
+in your effect monad.
+
+But note that it this module is intended to be used for implementing other
+more granular effects. For example, instead of running @putStrLn@ directly in
+the effect monad, consider defining a @Console@ effect:
+
+@
+data Console e ans = Console { print :: Op String () e ans }
+@
+
+Then you can use the functionality in this module to implement the effect
+handler:
+
+@
+console :: IOEff :? e => Eff (Console :* e) a -> Eff e a
+console = handler Console { print = function (performIO . putStrLn) }
+@
+-}
+module Control.Ev.Monad
+  (
+  -- * Monad effect
+    MEff(MEff)
+  , performM
+  , runMEff
+  -- * IO effect
+  , IOEff
+  , performIO
+  , runIOEff
+  ) where
+
+import           Control.Ev.Eff
+import           Control.Monad.IO.Class
+
+------------------
+-- Monad effect --
+------------------
+
+-- | An effect for embedding another monad @m@ into the effect monad.
+newtype MEff m e ans = MEff
+    { meff :: forall a. Op (m a) a e ans
+    }
+
+-- | Perform a different monadic action in the effect monad.
+performM :: MEff m :? e => m a -> Eff e a
+performM = perform meff
+
+-- | Run an effect monad with a single other monadic effect.
+--
+-- All other effects need to be handled before this function can be used to
+-- get the final result.
+runMEff :: Monad m => Eff (MEff m :* ()) a -> m a
+runMEff = runEff . handlerRet
+  pure
+  MEff { meff = operation $ \a k -> pure $ a >>= runEff . k }
+
+---------------
+-- IO effect --
+---------------
+
+-- | An effect for embedding @IO@ into the effect monad.
+type IOEff = MEff IO
+
+-- | Perform an @IO@ action in the effect monad.
+performIO :: IOEff :? e => IO a -> Eff e a
+performIO = performM
+
+-- | Run an effect monad that has a single @IO@ effect.
+--
+-- All other effects need to be handled before this function can be used to
+-- get the final result.
+runIOEff :: Eff (IOEff :* ()) a -> IO a
+runIOEff = runMEff
+
+instance IOEff :? e => MonadIO (Eff e) where
+  liftIO = performIO