On Wed, Mar 07, 2007 at 11:59:24PM +0000, Ross Paterson wrote:
I propose that we restructure and split the mtl into two packages:
Thank you, I was just thinking the same thing, but I would never have thought to publically challenge the mtl folks :)
mtl-base: a Haskell-98 package containing the monad transformers and non-overloaded versions of the operations, e.g.
Why can't we have: class MonadTrans t where lift :: Monad m => m a -> t m a looks perfectly H98 to me. both ghci-6.7.20070223 -fno-glasgow-exts and hugs +98 accept it.
mtl (depending on mtl-base): multi-parameter+FD type classes with instances for the transformers in mtl-base, e.g.
Would it also now be possible to get my metalift operation in mtl? class MonadTrans t where lift :: Monad m => m a -> t m a metalift :: (Monad m, Monad m') => (forall a. m1 a -> m2 a) -> t m a -> t m' b It is often useful in practice, for instance metalift lift can be used at type State s a -> StateT s IO a (a suprisingly common request on #haskell). Disclaimer: I have been able to write instances for all mtl transformers *except ContT*, and it seems plausible that ContT may force the class to be split. Stefan