On 2017-08-16 04:54 PM, Dmitriy Matrosov wrote:
And even sent the answer not to the list.. Huh.
I'm not sure I fully undestand your use case, but your examples can be handled by the rank2classes package (http://hackage.haskell.org/package/rank2classes).
I want to have a type with many records:
data Volume t = Volume { _volName :: t String , _volSize :: t Int }
showVolume :: (Show (t String), Show (t Int)) => Volume t -> String showVolume x = "Volume " ++ show (_volName x) ++ ", " ++ show (_volSize x)
with instances parametrized by some other type. E.g. i want to define a `Monoid` based on that other type properties:
instance Alternative t => Monoid (Volume t) where mempty = Volume {_volName = empty, _volSize = empty} x `mappend` y = Volume { _volName = _volName x <|> _volName y , _volSize = _volSize x <|> _volSize y }
instance Rank2.Apply Volume where x <*> y = Volume { _volName = _volName x `Rank2.apply` _volName y , _volSize = _volSize x `Rank2.apply` _volSize y } instance Rank2.Applicative Volume where pure x = Volume {_volName = x, _volSize = x} instance Alternative t => Monoid (Volume t) where mempty = Rank2.pure empty x `mappend` y = Rank2.liftA2 (<|>) x y