On Wed, Dec 7, 2011 at 5:48 AM, Dmitry Kulagin wrote:

I am still pretty new in Haskell, but this problem annoys me already.

If I define certain monad as a type synonym:

   type StateA a = StateT SomeState SomeMonad a

Then I can't declare new monad based on the synonym:

   type StateB a = StateT SomeOtherState StateA a

The only way I know to overcome is to declare StateA without `a':

   type StateA = StateT SomeState SomeMonad

But it is not always possible with existing code base.

I'm afraid my proposal doesn't make this work. You could perhaps define StateB, but when you expand in a type you get: StateB a = StateT SomeOtherState StateA a which has a partially applied StateA, and is rejected. The only way to make this work is to eta reduce StateA manually, or make GHC recognize when a synonym can be eta reduced in this way (which might be both possible and useful as a separate proposal). My extension fell within the liberal type synonym space, which says that if you have: F G where F and G are both synonyms, and G is partially applied, then it is okay as long as expansion of F (and any subsequent expansions) cause G to become fully applied. My extension of this is just to allow partial application inside aliases as long as it meets these same criteria. The reason to disallow partially applied type aliases is that they make inference pretty much impossible, unless you only infer them in very limited circumstances perhaps. And if you can't get inference of them, you probably need to start having explicit annotations to tell the type checker what you want to happen, which has some of its own complications with the way quantifiers work in GHC and such. It'd cascade into some thorny issues. Hopefully that covers all the other subsequent stuff folks have been talking about. -- Dan