Control/Monad/Signatures.hs

    {-# LANGUAGE CPP #-}
    #if __GLASGOW_HASKELL__ >= 702
    {-# LANGUAGE Safe #-}
    #endif
    #if __GLASGOW_HASKELL__ >= 706
    {-# LANGUAGE PolyKinds #-}
    #endif
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Monad.Signatures
    -- Copyright   :  (c) Ross Paterson 2012
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  R.Paterson@city.ac.uk
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- Signatures for monad operations that require specialized lifting.
    -- Each signature has a uniformity property that the lifting should satisfy.
    -----------------------------------------------------------------------------

    module Control.Monad.Signatures (
        CallCC, Catch, Listen, Pass
      ) where

    -- | Signature of the @callCC@ operation,
    -- introduced in "Control.Monad.Trans.Cont".
    -- Any lifting function @liftCallCC@ should satisfy
    --
    -- * @'lift' (f k) = f' ('lift' . k) => 'lift' (cf f) = liftCallCC cf f'@
    --
    type CallCC m a b = ((a -> m b) -> m a) -> m a

    -- | Signature of the @catchE@ operation,
    -- introduced in "Control.Monad.Trans.Except".
    -- Any lifting function @liftCatch@ should satisfy
    --
    -- * @'lift' (cf m f) = liftCatch ('lift' . cf) ('lift' f)@
    --
    type Catch e m a = m a -> (e -> m a) -> m a

    -- | Signature of the @listen@ operation,
    -- introduced in "Control.Monad.Trans.Writer".
    -- Any lifting function @liftListen@ should satisfy
    --
    -- * @'lift' . liftListen = liftListen . 'lift'@
    --
    type Listen w m a = m a -> m (a, w)

    -- | Signature of the @pass@ operation,
    -- introduced in "Control.Monad.Trans.Writer".
    -- Any lifting function @liftPass@ should satisfy
    --
    -- * @'lift' . liftPass = liftPass . 'lift'@
    --
    type Pass w m a =  m (a, w -> w) -> m a