[Haskell-cafe] Use cases of empty type classes

[Geraldus]

Thanks everybody.  I found this helpful (:

вт, 8 мар. 2016 г. в 19:58, Alex Rozenshteyn <rpglover64 at gmail.com>:

> You can use them to claim something about multiple type classes at once,
> with some advantages over using ConstraintKinds for the same purpose:
> https://www.reddit.com/r/haskell/comments/49em92/haskells_typeclasses_we_can_do_better/d0rwvi8?context=2
>
>
> On Tue, Mar 8, 2016 at 6:36 AM Adam Bergmark <adam at bergmark.nl> wrote:
>
>> I have two use cases for them
>>
>>
>>    - In fay-jquery there’s an empty Selectable class that has instances
>>    for all elements that can be passed to the jQuery function, there is no
>>    need for methods since we in the background just pass the object along, but
>>    we want to make sure we don’t pass something nonsensical.
>>    - In a rest API serving JSON you need ToJSON and FromJSON instances
>>    for each type, but since internal APIs may also use JSON it’s possible that
>>    you by accident pass a type that isn’t versioned and supposed to be
>>    included in the public API. There I can have an empty PublicApiType class
>>    with instances for all public types to give a type error if I pass
>>    something internal by mistake.
>>
>>
>> /Adam Bergmark
>>
>>
>> On Tue 08 Mar 2016 at 07:10 Tomas Tauber <Tomas Tauber
>> <Tomas+Tauber+%3Ctomtau at connect.hku.hk%3E>> wrote:
>>
>>> Hi everyone,
>>>
>>> I have one question. What are current use cases of type classes with no
>>> methods?
>>>
>>> I saw early uses in type-level programming (e.g. HList [1]).
>>> In the OO world, interfaces with no methods are called marker interfaces
>>> -- their use cases range from things that could be done with datatype
>>> generic programming in Haskell (e.g. serialization) to metadata annotations
>>> (e.g. RandomAccess [2]).
>>>
>>> Regards,
>>>
>>> Tomas Tauber
>>>
>>>
>>> [1] http://okmij.org/ftp/Haskell/HList-ext.pdf
>>> [2]
>>> https://docs.oracle.com/javase/8/docs/api/java/util/RandomAccess.html
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>>
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