MaybeT-monads-tf 0.2 → 0.2.0.1

raw patch · 2 files changed

+18/−77 lines, 2 filesPVP: major bump suggested

API removals or changes: PVP suggests a major version bump

API changes (from Hackage documentation)

Files

• Control/Monad/Maybe.hs +11/−75
• MaybeT-monads-tf.cabal +7/−2

@@ -24,89 +24,25 @@  module Control.Monad.Maybe (   MaybeT(..)-  -- * Limitations-  -- $Limitations    -- * Example   -- $MaybeExample   ) where -import Control.Applicative-import Control.Monad() import Control.Monad.Trans()-import Control.Monad.Cont-import Control.Monad.Fix()-import Control.Monad.Reader-import Control.Monad.State-import Control.Monad.Writer---- | A monad transformer which adds Maybe semantics to an existing monad.-newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }--instance (Functor m) => Functor (MaybeT m) where-  fmap f = MaybeT . fmap (fmap f) . runMaybeT--instance Applicative m => Applicative (MaybeT m) where-  pure = MaybeT . pure . Just-  MaybeT f <*> MaybeT x = MaybeT $ liftA2 (<*>) f x--instance (Monad m) => Monad (MaybeT m) where-  fail _ = MaybeT (return Nothing)-  return = lift . return-  x >>= f = MaybeT (runMaybeT x >>= maybe (return Nothing) (runMaybeT . f))--instance (Monad m) => MonadPlus (MaybeT m) where-  mzero = MaybeT (return Nothing)-  mplus x y = MaybeT $ do v <- runMaybeT x-                          case v of-                            Nothing -> runMaybeT y-                            Just _  -> return v---instance MonadTrans MaybeT where-  lift x = MaybeT (liftM Just x)--instance (MonadCont m) => MonadCont (MaybeT m) where-  -- Again, I hope this is correct.-  callCC f = MaybeT (callCC (\c -> runMaybeT (f (wrap c))))-    where wrap :: (Maybe a -> m (Maybe b)) -> a -> MaybeT m b-          wrap c = MaybeT . c . Just---- MonadError: MonadError has fairly weird semantics when lifted by MaybeT,--- so let's skip it for now.--instance (MonadIO m) => MonadIO (MaybeT m) where-  liftIO = lift . liftIO+import Control.Monad.Trans.Maybe+import Control.Monad.Fix  instance (MonadFix m) => MonadFix (MaybeT m) where-  -- I hope this is correct.  At a minimum, it typechecks.-  mfix f = MaybeT (mfix (maybe (return Nothing) (runMaybeT . f)))---- MonadList: Not implemented.--instance (MonadReader m) => MonadReader (MaybeT m) where-  type EnvType (MaybeT m) = EnvType m-  ask = lift ask-  local f m = MaybeT (local f (runMaybeT m))---- MonadRWS: Not implemented.---- Taken from http://www.haskell.org/haskellwiki/New_monads/MaybeT .--- altered for the type family version-instance MonadState m => MonadState (MaybeT m) where-  type StateType (MaybeT m) = StateType m-  get = lift get-  put = lift . put--instance (MonadWriter m) => MonadWriter (MaybeT m) where-  type WriterType (MaybeT m) = WriterType m-  tell = lift . tell-  listen m = MaybeT (listen (runMaybeT m) >>= (return . liftMaybe))-    where liftMaybe (Nothing, _) = Nothing-          liftMaybe (Just x,  w) = Just (x,w)-  -- I'm not sure this is useful, but it's the best I can do:-  pass m = MaybeT (runMaybeT m >>= maybe (return Nothing)-                                         (liftM Just . pass . return))+  -- Ref: http://www.haskell.org/pipermail/libraries/2011-April/016202.html+  mfix f = MaybeT (mfix (runMaybeT . f . unJust))+   where+    unJust (Just x) = x+    -- if f is strict then mfix produces bottom, so f must produce a+    -- constructor without inspecting its argument. if that's Nothing, f+    -- cannot inspect its argument at all; if it's Just, then the call to+    -- unJust is safe.+    unJust Nothing = error "MaybeT mfix: Nothing! (this should be impossible)"  {- $MaybeExample 

@@ -1,14 +1,19 @@ Cabal-Version: >= 1.6  Name:     MaybeT-monads-tf-Version:  0.2+Version:  0.2.0.1 Category: Control-Synopsis: MaybeT monad transformer compatible with monads-tf instead of mtl+Synopsis: MaybeT monad transformer compatible with monads-tf (deprecated)  Description:     Support for computations with failures. This is a fork of the MaybeT     package by Eric Kidd, but compatible with the type-family based monad     classes of the monads-tf package.+    .+    This package is deprecated: the MaybeT transformer exists in the+    transformers package nowadays, and the only advantage this package+    provides over that one is the presence of a MonadFix instance, but+    it's incorrect anyway.  Author:       Ben Millwood <haskell@benmachine.co.uk> Maintainer:   Ben Millwood <haskell@benmachine.co.uk>