Control/Monad/Cont/Class.hs

    {-# LANGUAGE CPP #-}
    {- |
    Module      :  Control.Monad.Cont.Class
    Copyright   :  (c) The University of Glasgow 2001,
                   (c) Jeff Newbern 2003-2007,
                   (c) Andriy Palamarchuk 2007
    License     :  BSD-style (see the file LICENSE)

    Maintainer  :  libraries@haskell.org
    Stability   :  experimental
    Portability :  portable

    [Computation type:] Computations which can be interrupted and resumed.

    [Binding strategy:] Binding a function to a monadic value creates
    a new continuation which uses the function as the continuation of the monadic
    computation.

    [Useful for:] Complex control structures, error handling,
    and creating co-routines.

    [Zero and plus:] None.

    [Example type:] @'Cont' r a@

    The Continuation monad represents computations in continuation-passing style
    (CPS).
    In continuation-passing style function result is not returned,
    but instead is passed to another function,
    received as a parameter (continuation).
    Computations are built up from sequences
    of nested continuations, terminated by a final continuation (often @id@)
    which produces the final result.
    Since continuations are functions which represent the future of a computation,
    manipulation of the continuation functions can achieve complex manipulations
    of the future of the computation,
    such as interrupting a computation in the middle, aborting a portion
    of a computation, restarting a computation, and interleaving execution of
    computations.
    The Continuation monad adapts CPS to the structure of a monad.

    Before using the Continuation monad, be sure that you have
    a firm understanding of continuation-passing style
    and that continuations represent the best solution to your particular
    design problem.
    Many algorithms which require continuations in other languages do not require
    them in Haskell, due to Haskell's lazy semantics.
    Abuse of the Continuation monad can produce code that is impossible
    to understand and maintain.
    -}

    module Control.Monad.Cont.Class (
        MonadCont(..),
      ) where

    import Control.Monad.Trans.Cont (ContT)
    import qualified Control.Monad.Trans.Cont as ContT
    import Control.Monad.Trans.Error as Error
    import Control.Monad.Trans.Except as Except
    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
    import Data.Monoid

    class Monad m => MonadCont m where
        {- | @callCC@ (call-with-current-continuation)
        calls a function with the current continuation as its argument.
        Provides an escape continuation mechanism for use with Continuation monads.
        Escape continuations allow to abort the current computation and return
        a value immediately.
        They achieve a similar effect to 'Control.Monad.Error.throwError'
        and 'Control.Monad.Error.catchError'
        within an 'Control.Monad.Error.Error' monad.
        Advantage of this function over calling @return@ is that it makes
        the continuation explicit,
        allowing more flexibility and better control
        (see examples in "Control.Monad.Cont").

        The standard idiom used with @callCC@ is to provide a lambda-expression
        to name the continuation. Then calling the named continuation anywhere
        within its scope will escape from the computation,
        even if it is many layers deep within nested computations.
        -}
        callCC :: ((a -> m b) -> m a) -> m a
    #if __GLASGOW_HASKELL__ >= 707
        {-# MINIMAL callCC #-}
    #endif

    instance MonadCont (ContT r m) where
        callCC = ContT.callCC

    -- ---------------------------------------------------------------------------
    -- Instances for other mtl transformers

    instance (Error e, MonadCont m) => MonadCont (ErrorT e m) where
        callCC = Error.liftCallCC callCC

    instance MonadCont m => MonadCont (ExceptT e m) where
        callCC = Except.liftCallCC callCC

    instance MonadCont m => MonadCont (IdentityT m) where
        callCC = Identity.liftCallCC callCC

    instance MonadCont m => MonadCont (ListT m) where
        callCC = List.liftCallCC callCC

    instance MonadCont m => MonadCont (MaybeT m) where
        callCC = Maybe.liftCallCC callCC

    instance MonadCont m => MonadCont (ReaderT r m) where
        callCC = Reader.liftCallCC callCC

    instance (Monoid w, MonadCont m) => MonadCont (LazyRWS.RWST r w s m) where
        callCC = LazyRWS.liftCallCC' callCC

    instance (Monoid w, MonadCont m) => MonadCont (StrictRWS.RWST r w s m) where
        callCC = StrictRWS.liftCallCC' callCC

    instance MonadCont m => MonadCont (LazyState.StateT s m) where
        callCC = LazyState.liftCallCC' callCC

    instance MonadCont m => MonadCont (StrictState.StateT s m) where
        callCC = StrictState.liftCallCC' callCC

    instance (Monoid w, MonadCont m) => MonadCont (LazyWriter.WriterT w m) where
        callCC = LazyWriter.liftCallCC callCC

    instance (Monoid w, MonadCont m) => MonadCont (StrictWriter.WriterT w m) where
        callCC = StrictWriter.liftCallCC callCC