[Haskell-cafe] More liberal than liberal type synonyms

[Yves Parès]

Ah, maybe Dan could tell us if it works only with GHC 7.

Dmitry, I had your problem many times. The last time was when I saw you
could define the ContT monad in terms of Cont (the opposite is done in the
mtl).
It leads to a simpler code, but you are stucked when trying to define ContT
as an instance of MonadTrans:

data Cont r a = ...
-- [instances of Monad Cont, blah blah blah]

type ContT r m a = Cont r (m a)

instance MonadTrans (ContT r) where  -- *This doesn't compile*, even if it
is logical
  lift = ...

For short, type synonyms work for mere aliases, but not for full-fledged
type-level non-inductive functions.
And sometimes we intuitively want to use them as such.

2011/12/7 Dmitry Kulagin <dmitry.kulagin at gmail.com>

> > 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 <limestrael at 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 <kolsrud at 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.kulagin at 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.doel at 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 at haskell.org
> >>> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >>>
> >>> _______________________________________________
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> >>
> >>
> >>
> >>
> >> --
> >> Mvh Øystein Kolsrud
> >>
> >> _______________________________________________
> >> Haskell-Cafe mailing list
> >> Haskell-Cafe at haskell.org
> >> http://www.haskell.org/mailman/listinfo/haskell-cafe
> >>
> >
>
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