On Thu, Mar 8, 2012 at 2:43 AM, Ross Paterson
Seeking views before a new major release of transformers package. The docs are here:
http://code.haskell.org/~ross/transformers/dist/doc/html/transformers/
The source is here:
darcs get http://code.haskell.org/~ross/transformers
The major changes from version 0.2.2.0 are:
* Foldable and Traversable instances for transformers that support them. * extra Monad instances:
instance (MonadFix m) => MonadFix (MaybeT m) instance (MonadFix m) => MonadFix (IdentityT m) instance (Monad f, Monad g) => Monad (Product f g) instance (MonadPlus f, MonadPlus g) => MonadPlus (Product f g) instance (MonadFix f, MonadFix g) => MonadFix (Product f g)
* new functors Backwards and Reverse * a new Lift transformer, a generalization of Errors * generalized constructor functions:
state :: Monad m => (s -> (a, s)) -> StateT s m a reader :: Monad m => (r -> a) -> ReaderT r m a writer :: Monad m => (a, w) -> WriterT w m a
Another issue that has been raised is: should the instance
instance Monad (ContT r m)
have a Monad constraint so that it can define fail?
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I doubt that this change could actually be merged into transformers, since it requires either FunDeps or Type Families, but I thought I'd mention it anyway. In Yesod, we have the monads Handler and Widget, which are essentially: newtype Handler a = Handler (ReaderT HandlerData IO a) newtype Widget a = Widget (WriterT WidgetData Handler a) We could in theory make the underlying monad a type variable as well, but this would produce confusing type signatures and error messages[1], as well as falsely give the impression that it would be valid to use different monads as the base for each of these. The result? We have something which is essentially a transformer, but actually isn't. Therefore, even though we *want* to have a `lift` function, we can't define a `MonadTrans` instance. My solution was to create a new typeclass[2]: class MonadLift base m | m -> base where lift :: base a -> m a It's simple to automatically make all instances of MonadTrans an instance of MonadLift: instance (Monad m, MonadTrans t) => MonadLift m (t m) where lift = Control.Monad.Trans.Class.lift and still make separate instances for Handler and Widget. As I said, I'm not really trying to push this into transformers, but I thought I would mention it. I think being able to make non-MonadTrans transformers can often be a good API design, and it would be nice to support it in the libraries. Michael [1] The type aren't quite as simple as I've presented them here. [2] http://hackage.haskell.org/packages/archive/yesod-core/0.10.2.1/doc/html/Yes...