Using polymorphic type in type family in haskell -
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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