[Haskell-cafe] More liberal than liberal type synonyms

[Dmitry Kulagin]

> 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
>>> >
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>>
>>
>>
>>
>> --
>> Mvh Øystein Kolsrud
>>
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