[Haskell-cafe] More liberal than liberal type synonyms

[Dan Doel]

On Wed, Dec 7, 2011 at 5:48 AM, Dmitry Kulagin <dmitry.kulagin at gmail.com> 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