> Dmitry, does your code work with LiberalTypeSynonyms extention activated? No, the same error: Type synonym `StateA' should have 1 argument, but has been given 0
But I have GHC 6.12.3 Dmitry 2011/12/7 Yves Parès <limestr...@gmail.com>: > This is impossible: > in the definition of 'StateT s m a', m must be a monad and then have the * > -> * kind. > So you cannot pass (StateA a), because it has simply the * kind. > > Dmitry, does your code work with LiberalTypeSynonyms extention activated? > > > 2011/12/7 Øystein Kolsrud <kols...@gmail.com> >> >> You should be able to write something like this: >> >> type StateB a b = StateT SomeOtherState (StateA a) b >> >> Best regards, Øystein Kolsrud >> >> >> On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin <dmitry.kula...@gmail.com> >> 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 <dan.d...@gmail.com> 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 >>> > >>> > _______________________________________________ >>> > Haskell-Cafe mailing list >>> > Haskell-Cafe@haskell.org >>> > http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe@haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >> >> >> -- >> Mvh Øystein Kolsrud >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe@haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
