[Haskell-cafe] Datakind and instance

[Georgi Lyubenov]

You can make it polykinded:


{-# LANGAUGE PolyKinds #-}

class HasIniConfig (a :: k) where
   getConfig ...

And then you can use both types of kind ProjectionType, and also 
"regular types" of kind Type.

On 6/13/24 18:32, PICCA Frederic-Emmanuel wrote:
> Hello,
>
> I have a class
>
> class HasIniConfig where
>     getConfig ...
>
> a kind using DataKind
>
> data ProjectionType = TypeA
>                      | TypeB
>                      ...
>
> I would like to create instance for each 'TypeX
>
> But when I try to write an instance
>
> I get this error
>
> src/Hkl/Binoculars/Projections/QCustom.hs:195:23: error:
>      • Expected a type, but
>        ‘'QCustomProjection’ has kind
>        ‘ProjectionType’
>      • In the first argument of ‘HasIniConfig’, namely
>          ‘'QCustomProjection’
>        In the instance declaration for ‘HasIniConfig 'QCustomProjection’
>      |
> 195 | instance HasIniConfig 'QCustomProjection where
>      |                       ^^^^^^^^^^^^^^^^^^
>
>
> I should add class HasIniConfig (a :: ProjectionType) where
>
> But in that cas I can not  create instance for other type, and I need to :)
>
> I would be glade if someone could explain how to all this.
>
> Thanks
>
> Frederic
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