It seems really hard to solve this, since the type checker works before instance selection has had a chance to do anything.

Instead of looking at the instance declaration, look at the use site:

   lift x

expects the argument to have type

   x :: t m a

for some t :: (* -> *) -> * -> *, m :: * -> *, and a :: *; it's not until t is known that we can do instance selection, and in your case, EitherT M A B doesn't have the required form and so is a type error already.

I think the best answer is sadly to just have to have a (kind-polymorphic!) newtype flip and deal with it.

I can imagine there being some way to (ab)use kind polymorphism to redefine MonadTrans in a way that allows the 'm' argument to appear in more places in the target type, but I'm not clever enough to come up with a proposal for how to do so.

  -- ryan


On Mon, Aug 13, 2012 at 4:38 PM, Tony Morris <tonymorris@gmail.com> wrote:
I have a data-type that is similar to EitherT, however, I have ordered
the type variables like so:

data EitherT (f :: * -> *) (a :: *) (b :: *) = ...

This allows me to declare some desirable instances:

instance Functor f => Bifunctor (EitherT f)
instance Foldable f => Bifoldable (EitherT f)
instance Traversable f => Bitraversable (EitherT f)

However, I am unable to declare a MonadTrans instance:

instance MonadTrans (EitherT a) -- kind error

I looked at Control.Compose.Flip to resolve this, but it does not appear
to be kind-polymorphic.
http://hackage.haskell.org/packages/archive/TypeCompose/0.9.1/doc/html/src/Control-Compose.html#Flip

I was wondering if there are any well-developed techniques to deal with
this? Of course, I could just write my own Flip with the appropriate
kinds and be done with it. Maybe there is a more suitable way?


--
Tony Morris
http://tmorris.net/



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