-- Hoogle documentation, generated by Haddock -- See Hoogle, http://www.haskell.org/hoogle/ -- | Generalizing lift to monad stacks -- @package MonadStack @version 0.1.0.3 -- | MonadStack provides a multi-level lift function, called -- liftFrom. It allows lifting from an arbitrary monad in a -- monad transformer stack up to any higher monad in that stack. And -- example use might be: -- -- 
--   newtype MyMonad = MyMonad ... deriving (Monad)
--   
--   doFoo :: MonadStack MyMonad m => something -> m whatever
--   doFoo arg = liftFrom go
--       where
--           go :: MyMonad whatever
--           go = do ...
--
-- -- This allows calling doFoo either in the MyMonad monad, or in any monad -- transformer stack on top of the MyMonad. -- -- MonadStack is similar to the Control.Monad.Base module from -- the transformers-base package, except that there is no -- functional dependency between the monads. A monad transformer stack -- can only have one base, but it can have several MonadStack -- implementations. -- -- MonadStack is very similar to the Control.Monad.Lifter module -- from the MonadCompose package, with two exceptions. One, -- MonadStack does not require the OverlappingInstances and -- UndecidableInstances extensions- both of which are prone to generating -- hard to diagnose bugs. But this implies two, that MonadStack is much -- less feature-rich than Lifter is. Specifically, MonadStack only works -- with "proper" monad transformers (those that implment -- MonadTrans), while Lifter generalizes the notion of lifting -- and works with more different types of monads, especially -- MonadPlus. module Control.Monad.MonadStack -- | A multi-level lifter class. -- -- The existance of an implementation of MonadStack m n implies -- that is is possible to convert an m a into an n a -- via zero or more applications of lift. class MonadStack m n liftFrom :: MonadStack m n => m a -> n a -- | The base case implementation -- -- You can always convert an m a into an m a by -- applying 0 lifts. This is like treating an m a as a -- zero-level monad transformer stack (a monad transformer stack with no -- monad transformers), and liftFrom is just the id -- function. -- | The inductive step implementation -- -- If there exists an implementation of MonadStack k m for -- monads k and m (that is, we can perform a -- liftFrom to convert a k a to a m a), and n -- is a monad transformer, then we can construct an implementation of -- MonadStack k (n m), by just adding one more lift to -- the liftFrom. instance (Monad k, Monad m, MonadTrans n, MonadStack k m) => MonadStack k (n m) instance MonadStack m m