Re: [Haskell-cafe] Class instances on class constraints

28 Jul 2010


      2010/7/28 Dušan Kolář :
...
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.

Re: [Haskell-cafe] Class instances on class constraints

Felipe Lessa