2010/7/28 Dušan Kolář <ko...@fit.vutbr.cz>:
> which does not work, of course (Flexible or Undecidable instances won't
> help). The aim is to have addElem function that works differently according
> to situation whether a type, which is base of the list/set, is a member of
> class Eq or Ord. Could you point me or hint me how to get as close as
> possible to the required solution? Maybe I'm not able to see an obvious
> way...
Use the good'ol trick:
newtype ByEq a = ByEq {unByEq :: a}
newtype ByOrd a = ByOrd {unByOrd :: a}
instance (Eq a) => SetOL (ByEq a) where
addElem = addEq . unByEq
instance (Ord a) => SetOL (ByOrd a) where
addElem = addOrd . unByOrd
Cheers,
--
Felipe.
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