repro.hs

data PosInf a = Infinity | Num a deriving (Show, Read, Eq)

instance Ord a => Ord (PosInf a) where
  compare Infinity Infinity = EQ
  compare Infinity Num{} = GT
  compare Num{} Infinity = LT
  compare (Num a) (Num b) = compare a b

pInfty :: PosInf a
pInfty = Infinity

give_it_a_try :: PosInf a => a -> a
give_it_a_try a b
  | a == Infinity || b == Infinity -> Infinity
  | otherwise = a + b