Control/Monad/Error/Class.hs

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE FlexibleInstances #-}
    {-# LANGUAGE FunctionalDependencies #-}
    {-# LANGUAGE MultiParamTypeClasses #-}
    {-# LANGUAGE UndecidableInstances #-}

    {- |
    Module      :  Control.Monad.Error.Class
    Copyright   :  (c) Michael Weber <michael.weber@post.rwth-aachen.de> 2001,
                   (c) Jeff Newbern 2003-2006,
                   (c) Andriy Palamarchuk 2006
                   (c) Edward Kmett 2012
    License     :  BSD-style (see the file LICENSE)

    Maintainer  :  libraries@haskell.org
    Stability   :  experimental
    Portability :  non-portable (multi-parameter type classes)

    [Computation type:] Computations which may fail or throw exceptions.

    [Binding strategy:] Failure records information about the cause\/location
    of the failure. Failure values bypass the bound function,
    other values are used as inputs to the bound function.

    [Useful for:] Building computations from sequences of functions that may fail
    or using exception handling to structure error handling.

    [Zero and plus:] Zero is represented by an empty error and the plus operation
    executes its second argument if the first fails.

    [Example type:] @'Either' 'String' a@

    The Error monad (also called the Exception monad).
    -}

    {-
      Rendered by Michael Weber <mailto:michael.weber@post.rwth-aachen.de>,
      inspired by the Haskell Monad Template Library from
        Andy Gill (<http://web.cecs.pdx.edu/~andy/>)
    -}
    module Control.Monad.Error.Class (
        Error(..),
        MonadError(..),
      ) where

    import Control.Monad.Trans.Except (Except, ExceptT)
    import Control.Monad.Trans.Error (Error(..), ErrorT)
    import qualified Control.Monad.Trans.Except as ExceptT (throwE, catchE)
    import qualified Control.Monad.Trans.Error as ErrorT (throwError, catchError)
    import Control.Monad.Trans.Identity as Identity
    import Control.Monad.Trans.List as List
    import Control.Monad.Trans.Maybe as Maybe
    import Control.Monad.Trans.Reader as Reader
    import Control.Monad.Trans.RWS.Lazy as LazyRWS
    import Control.Monad.Trans.RWS.Strict as StrictRWS
    import Control.Monad.Trans.State.Lazy as LazyState
    import Control.Monad.Trans.State.Strict as StrictState
    import Control.Monad.Trans.Writer.Lazy as LazyWriter
    import Control.Monad.Trans.Writer.Strict as StrictWriter

    import Control.Monad.Trans.Class (lift)
    import Control.Exception (IOException, catch, ioError)
    import Control.Monad

    #if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ < 707
    import Control.Monad.Instances ()
    #endif

    import Data.Monoid
    import Prelude (Either(..), (.), IO)

    {- |
    The strategy of combining computations that can throw exceptions
    by bypassing bound functions
    from the point an exception is thrown to the point that it is handled.

    Is parameterized over the type of error information and
    the monad type constructor.
    It is common to use @'Data.Either' String@ as the monad type constructor
    for an error monad in which error descriptions take the form of strings.
    In that case and many other common cases the resulting monad is already defined
    as an instance of the 'MonadError' class.
    You can also define your own error type and\/or use a monad type constructor
    other than @'Either' 'String'@ or @'Either' 'IOError'@.
    In these cases you will have to explicitly define instances of the 'MonadError'
    class.
    (If you are using the deprecated "Control.Monad.Error" or
    "Control.Monad.Trans.Error", you may also have to define an 'Error' instance.)
    -}
    class (Monad m) => MonadError e m | m -> e where
        -- | Is used within a monadic computation to begin exception processing.
        throwError :: e -> m a

        {- |
        A handler function to handle previous errors and return to normal execution.
        A common idiom is:

        > do { action1; action2; action3 } `catchError` handler

        where the @action@ functions can call 'throwError'.
        Note that @handler@ and the do-block must have the same return type.
        -}
        catchError :: m a -> (e -> m a) -> m a
    #if __GLASGOW_HASKELL__ >= 707
        {-# MINIMAL throwError, catchError #-}
    #endif

    instance MonadError IOException IO where
        throwError = ioError
        catchError = catch

    -- ---------------------------------------------------------------------------
    -- Our parameterizable error monad

    instance MonadError e (Either e) where
        throwError             = Left
        Left  l `catchError` h = h l
        Right r `catchError` _ = Right r

    instance (Monad m, Error e) => MonadError e (ErrorT e m) where
        throwError = ErrorT.throwError
        catchError = ErrorT.catchError

    instance Monad m => MonadError e (ExceptT e m) where
        throwError = ExceptT.throwE
        catchError = ExceptT.catchE

    -- ---------------------------------------------------------------------------
    -- Instances for other mtl transformers
    --
    -- All of these instances need UndecidableInstances,
    -- because they do not satisfy the coverage condition.

    instance MonadError e m => MonadError e (IdentityT m) where
        throwError = lift . throwError
        catchError = Identity.liftCatch catchError

    instance MonadError e m => MonadError e (ListT m) where
        throwError = lift . throwError
        catchError = List.liftCatch catchError

    instance MonadError e m => MonadError e (MaybeT m) where
        throwError = lift . throwError
        catchError = Maybe.liftCatch catchError

    instance MonadError e m => MonadError e (ReaderT r m) where
        throwError = lift . throwError
        catchError = Reader.liftCatch catchError

    instance (Monoid w, MonadError e m) => MonadError e (LazyRWS.RWST r w s m) where
        throwError = lift . throwError
        catchError = LazyRWS.liftCatch catchError

    instance (Monoid w, MonadError e m) => MonadError e (StrictRWS.RWST r w s m) where
        throwError = lift . throwError
        catchError = StrictRWS.liftCatch catchError

    instance MonadError e m => MonadError e (LazyState.StateT s m) where
        throwError = lift . throwError
        catchError = LazyState.liftCatch catchError

    instance MonadError e m => MonadError e (StrictState.StateT s m) where
        throwError = lift . throwError
        catchError = StrictState.liftCatch catchError

    instance (Monoid w, MonadError e m) => MonadError e (LazyWriter.WriterT w m) where
        throwError = lift . throwError
        catchError = LazyWriter.liftCatch catchError

    instance (Monoid w, MonadError e m) => MonadError e (StrictWriter.WriterT w m) where
        throwError = lift . throwError
        catchError = StrictWriter.liftCatch catchError