Using polymorphic type in type family in haskell -

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i'm learning type family, it's confusing. when define polymorphic type outside of class definition, works well.

{-# language rank2types #-} type t = num =>

but when polymorphic type defined inside of class definition, 

{-# language typefamilies #-} {-# language rank2types #-}  data d = d  class a     type t :: * instance d     type t d = num =>

then complier shows error :

illegal polymorphic or qualified type: forall a. num => in type instance declaration 't' in instance declaration 'a d'

is there way make function in class return type polymorphic, 3(num => a)?


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