[Haskell-cafe] More liberal than liberal type synonyms

[Yves Parès]

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