EitherT (empty) → 0.0.1

raw patch · 4 files changed

+102/−0 lines, 4 filesdep +basedep +mtlsetup-changed

Dependencies added: base, mtl

Files

• EitherT.cabal +15/−0
• LICENSE +26/−0
• Setup.hs +2/−0
• src/Control/Monad/Either.hs +59/−0

@@ -0,0 +1,15 @@+Name:                EitherT+Version:             0.0.1+Synopsis:            EitherT monad transformer+Description:         Support for computations with informative failures.+License:             BSD3+License-file:        LICENSE+Category:            Control+Author:              Eyal Lotem <eyal.lotem+hackage@gmail.com>+Maintainer:          Eyal Lotem <eyal.lotem+hackage@gmail.com>+Stability:           experimental+Build-Type:          Simple+HS-Source-Dirs:      src+Build-Depends:       base >= 4 && < 5, mtl+Exposed-modules:     Control.Monad.Either+ghc-options:         -Wall 

@@ -0,0 +1,26 @@+EitherT monad transformer.+Copyright 2007 Eyal Lotem.  All rights reserved.++Redistribution and use in source and binary forms, with or without+modification, are permitted provided that the following conditions are met:++  * Redistributions of source code must retain the above copyright notice,+    this list of conditions and the following disclaimer.+  * Redistributions in binary form must reproduce the above copyright+    notice, this list of conditions and the following disclaimer in the+    documentation and/or other materials provided with the distribution.+  * The names of this library's contributors may not be used to endorse or+    promote products derived from this software without specific prior+    written permission.++THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE+POSSIBILITY OF SUCH DAMAGE.

@@ -0,0 +1,2 @@+import Distribution.Simple+main = defaultMain

@@ -0,0 +1,59 @@+{-# OPTIONS -Wall -O2 -fno-warn-orphans #-}+{-# LANGUAGE ScopedTypeVariables #-}++-- Exports Applicative/Monad instances for Either+module Control.Monad.Either(EitherT(..), left) where++import Control.Monad(liftM)+import Control.Monad.Trans(MonadTrans(..))+import Control.Monad.Instances()+import Control.Monad.Trans(MonadIO(..))+import Control.Applicative(Applicative(..), liftA2)++newtype EitherT l m a = EitherT { runEitherT :: m (Either l a) }+inEitherT0 :: m (Either l a) -> EitherT l m a+inEitherT0 = EitherT+inEitherT1 :: (m (Either l a) -> m (Either l b)) ->+              EitherT l m a -> EitherT l m b+inEitherT1 f = inEitherT0 . f . runEitherT+inEitherT2 :: (m (Either l a) -> m (Either l b) -> m (Either l c)) ->+              EitherT l m a -> EitherT l m b -> EitherT l m c+inEitherT2 f = inEitherT1 . f . runEitherT++left :: Monad m => l -> EitherT l m a+left = EitherT . return . Left++instance Monad m => Monad (EitherT l m) where+  -- We can't support "fail" because we don't have a+  -- (String -> l). But we can at least make it a Left, with the error inside+  -- it as a pure exception.+  fail = EitherT . return . Left . error+  return = EitherT . return . Right+  EitherT x >>= f = EitherT $ do+    res <- x+    case res of+      Right r -> runEitherT . f $ r+      Left l -> return (Left l)++instance MonadTrans (EitherT l) where+  lift = EitherT . liftM Right++instance Applicative (Either l) where+  pure = Right+  Right f <*> Right x = Right (f x)+  Left e <*> _ = Left e+  _ <*> Left e = Left e++instance Monad (Either l) where+  return = pure+  Right x >>= f = f x+  Left e >>= _ = Left e++instance Functor f => Functor (EitherT l f) where+  fmap = inEitherT1 . fmap . fmap+instance Applicative f => Applicative (EitherT l f) where+  pure = inEitherT0 . pure . pure+  (<*>) = inEitherT2 . liftA2 . liftA2 $ id++instance (MonadIO m) => MonadIO (EitherT l m) where+  liftIO = lift . liftIO