Re: [Haskell-cafe] More liberal than liberal type synonyms

You should be able to write something like this:

type StateB a b = StateT SomeOtherState (StateA a) b

Thank you for reply, but this variant actually does not compile: StateA and (StateA a) have different kinds. Dmitry

Best regards, Øystein Kolsrud

On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin wrote:

...

Hi Dan,

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 am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described.

Thanks! Dmitry

On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel wrote:

...

Greetings,

In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows:

   type Foo a b = b -> a    type Bar f = f String Int

   baz :: Bar Foo    baz = show

because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC:

   type Foo a b = b -> a    type Bar f = f (Foo Int) (Foo Int)    type Baz f g = f Int -> g Int

   quux :: Bar Baz    quux = id

That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use.

I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up.

-- Dan

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-- Mvh Øystein Kolsrud