restructuring the mtl

Ross Paterson ross at
Wed Mar 7 18:59:24 EST 2007

I propose that we restructure and split the mtl into two packages:

mtl-base: a Haskell-98 package containing the monad transformers
and non-overloaded versions of the operations, e.g.

	module Control.Monad.Trans.State where

	newtype StateT s m a = StateT { runStateT :: s -> m (a,s) }

	type State s = StateT s Identity

	instance (Functor m) => Functor (StateT s m)
	instance (Monad m) => Monad (StateT s m)
	instance (MonadPlus m) => MonadPlus (StateT s m)
	instance (MonadFix m) => MonadFix (StateT s m)

	get :: (Monad m) => StateT s m s
	put :: (Monad m) => s -> StateT s m ()

	liftStateT :: Monad m => m a -> StateT s m a

mtl (depending on mtl-base): multi-parameter+FD type classes with
instances for the transformers in mtl-base, e.g.

	module Control.Monad.State where

	import qualified Control.Monad.Trans.Error as Error
	import qualified Control.Monad.Trans.Reader as Reader
	import qualified Control.Monad.Trans.State as State
	import qualified Control.Monad.Trans.Writer as Writer

	class (Monad m) => MonadState s m | m -> s where
		get :: m s
		put :: s -> m ()

	instance (Monad m) => MonadState s (StateT s m)

	instance (Error e, MonadState s m) => MonadState s (ErrorT e m)
	instance (MonadState s m) => MonadState s (ReaderT r m)
	instance (Monoid w, MonadState s m) => MonadState s (WriterT w m)

One benefit is that it would be possible to use monad transformers in
portable programs, at the cost of a little explicit lifting of operations.
Often when I use a stack of monad transformers, I define aliases for the
new monad and its operations, so this wouldn't be much extra effort.

A second benefit is that one could introduce other packages with other
interfaces, e.g. one using associated types.

The revised mtl would be almost compatible with the existing one, except
1) The monad transformer and corresponding monad would have the same
   strictness (this has already been done in the HEAD).
2) It wouldn't be possible to declare instances for the corresponding monad.

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