MaybeT (empty) → 0.1.0

raw patch · 4 files changed

+197/−0 lines, 4 filesdep +basedep +mtlbuild-type:Customsetup-changed

Dependencies added: base, mtl

Files

Control/Monad/Maybe.hs +156/−0
LICENSE +26/−0
MaybeT.cabal +13/−0
Setup.hs +2/−0

+ Control/Monad/Maybe.hs view

@@ -0,0 +1,156 @@+{-# LANGUAGE MultiParamTypeClasses, UndecidableInstances #-}++{- |+Copyright    : 2007 Eric Kidd+License      : BSD3+Stability    : experimental+Portability  : non-portable (multi-parameter type classes, undecidable instances)++The 'MaybeT' monad.  See+<http://www.haskell.org/haskellwiki/New_monads/MaybeT> for more widely-used+version.  Our 'Functor' instance and our implementation of '>>=' are+borrowed from there.++[Computation type:] Computations which may fail or return nothing.++[Binding strategy:] Failure returns the value 'Nothing', bypassing any+bound functions which follow.  Success returns a value wrapped in 'Just'.++[Useful for:] Building computations from steps which may fail.  No error+information is returned.  (If error information is required, see+'Control.Monad.Error'.)++-}++module Control.Monad.Maybe (+  MaybeT(..)+  -- * Limitations+  -- $Limitations++  -- * Example+  -- $MaybeExample+  ) where++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 (Monad m) => Monad (MaybeT m) where+  fail _ = MaybeT (return Nothing)+  return = lift . return+  x >>= f = MaybeT (runMaybeT x >>= maybe (return Nothing) (runMaybeT . f))++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++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.++-- MonadPlus: Ambiguous.  See note in introduction.++-- Requires -fallow-undecidable-instances.+instance (MonadReader r m) => MonadReader r (MaybeT m) where+  ask = lift ask+  local f m = MaybeT (local f (runMaybeT m))++-- MonadRWS: Not implemented.++-- Taken from http://www.haskell.org/haskellwiki/New_monads/MaybeT .+instance MonadState s m => MonadState s (MaybeT m) where+  get = lift get+  put = lift . put++-- Requires -fallow-undecidable-instances.+instance (MonadWriter w m) => MonadWriter w (MaybeT m) where+  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))++{- $Limitations++The instance @MonadPlus@ is not provided, because it has ambiguous+semantics.  It could refer to either++>instance MonadPlus m => MonadPlus (MaybeT m)++...lifting the semantics of an underlying 'MaybeT' monad, or++>instance MonadPlus (MaybeT m)++...with semantics similar to @MonadPlus Maybe@.++-}++{- $MaybeExample++Here is an example that shows how to use 'MaybeT' to propagate an+end-of-file condition in the IO monad.  In the example below, both+@maybeReadLine@ and @failIfQuit@ may cause a failure, which will propagate+out to @main@ without further intervention.++>import System.Console.Readline+>import Data.Maybe+>import Control.Monad+>import Control.Monad.Trans+>import Control.Monad.Maybe+>+>-- 'MaybeIO' is the type of computations which do IO, and which may fail.+>type MaybeIO = MaybeT IO+>+>-- 'readline' already has type 'String -> IO (Maybe String)'; we just need+>-- to wrap it.+>maybeReadLine :: String -> MaybeIO String+>maybeReadLine prompt = MaybeT (readline prompt)+>+>-- Fail if 'str' equals "quit".+>failIfQuit :: (Monad m) => String -> m ()+>failIfQuit str = when (str == "quit") (fail "Quitting")+>+>-- This task may fail in several places.  Try typing Control-D or "quit" at+>-- any prompt.+>concatTwoInputs :: MaybeIO ()+>concatTwoInputs = do+>  s1 <- maybeReadLine "String 1> "+>  failIfQuit s1+>  s2 <- maybeReadLine "String 2> "+>  failIfQuit s2+>  liftIO (putStrLn ("Concatenated: " ++ s1 ++ s2))+>+>-- Loop until failure.+>main :: IO ()+>main = do+>  result <- runMaybeT concatTwoInputs+>  if isNothing result+>    then putStrLn "Bye!"+>    else main++-}

+ LICENSE view

@@ -0,0 +1,26 @@+MaybeT monad transformer.+Copyright 2007 Eric Kidd.  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.

+ MaybeT.cabal view

@@ -0,0 +1,13 @@+Name:                MaybeT+Version:             0.1.0+Synopsis:            MaybeT monad transformer+Description:         Support for computations with failures.+License:             BSD3+License-file:        LICENSE+Category:            Control+Author:              Eric Kidd <haskell@randomhacks.net>+Maintainer:          Eric Kidd <haskell@randomhacks.net>+Stability:           experimental+Build-Depends:       base, mtl+Exposed-modules:     Control.Monad.Maybe+ghc-options:         -Wall -O

+ Setup.hs view

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