MonadCompose 0.3.0.0 → 0.4.0.0

raw patch · 2 files changed

+73/−4 lines, 2 filesdep ~base

Dependency ranges changed: base

Files

• Control/Monad/Lifter.hs +69/−0
• MonadCompose.cabal +4/−4

@@ -0,0 +1,69 @@+{-# LANGUAGE IncoherentInstances, FlexibleInstances, FlexibleContexts, MultiParamTypeClasses, TypeOperators, GeneralizedNewtypeDeriving, FunctionalDependencies, UndecidableInstances #-}
+
+module Control.Monad.Lifter where
+
+import Control.Monad.Plus
+import Control.Monad.Trans
+import Control.Monad.ST
+import Control.Monad.Identity
+import Control.Monad.Reader
+import Control.Applicative
+import Control.Monad.Morph
+
+-- | An automatic lifter. The idea of automatic lifting is due to Dan Piponi.
+class Lifter m n where
+	lift' :: m t -> n t
+
+instance Lifter (ST RealWorld) IO where
+	lift' = stToIO
+
+instance Lifter IO IO where
+	lift' = id
+
+instance Lifter (ST s) (ST s) where
+	lift' = id
+
+instance Lifter Identity Identity where
+	lift' = id
+
+instance (MonadTrans n, Monad x, Lifter m x) => Lifter m (n x) where
+	lift' = lift . lift'
+
+instance (Monad x, MFunctor m) => Lifter (m Identity) (m x) where
+	lift' = hoist (return . runIdentity)
+
+instance (Lifter m x) => Lifter m (x ::+ n) where
+	lift' = inl . lift'
+
+-- | Declared lifts.
+newtype Lifted m t = Lifted { runLifted :: m t } deriving (Monad, Applicative, Functor)
+
+instance MFunctor Lifted where
+	hoist f (Lifted m) = Lifted (f m)
+
+class Convert m n | m -> n where
+	convert :: m t -> Lifted n t
+
+instance Convert (Lifted m) m where
+	convert = id
+
+instance (Lifter IO n) => Convert IO n where
+	convert = Lifted . lift'
+
+instance (Lifter (ST s) n) => Convert (ST s) n where
+	convert = Lifted . lift'
+
+instance (Lifter (ReaderT r m) n) => Convert (ReaderT r m) n where
+	convert = Lifted . lift'
+
+infixl 1 >-=
+infixl 1 >-
+
+-- Explicitly declared lifts are needed here, so I can declare a special-case lift for
+-- Lifted. Lifted and its special case are needed so that I know how to unify types
+-- through nested uses of >-=.
+--
+-- | Auto-lifting versions of monadic bind.
+m >-= f = convert m >>= convert . f
+
+m >- m2 = convert m >> convert m2

@@ -1,7 +1,7 @@ name:                MonadCompose
-version:             0.3.0.0
+version:             0.4.0.0
 synopsis:            Methods for composing monads.
-description:         Includes basically the known ways of composing monads. Also includes an IO monad transformer.
+description:         Methods for composing monads, including an IO monad transformer.
 homepage:            http://alkalisoftware.net
 license:             BSD3
 license-file:        LICENSE
@@ -13,6 +13,6 @@ cabal-version:       >=1.8
 
 library
-  exposed-modules:     Control.Monad.IOT, Control.Monad.Distributive, Control.Monad.Plus
+  exposed-modules:     Control.Monad.IOT, Control.Monad.Distributive, Control.Monad.Plus, Control.Monad.Lifter
   -- other-modules: 
-  build-depends:       base ==4.6.*, ghc-prim ==0.3.*, mtl ==2.1.*, mmorph ==1.0.*, monad-products, transformers, MaybeT
+  build-depends:       base >=4 && <=5, ghc-prim ==0.3.*, mtl ==2.1.*, mmorph ==1.0.*, monad-products, transformers, MaybeT