Overloading.hs

class CustomClass a where
  add :: a -> a -> a
  mul :: a -> a -> a
  
newtype ZModZ7 = ZModZ7 Integer deriving (Eq, Show)

instance CustomClass ZModZ7 where
  add a b = mod (a + b) 7       -- Here, 'a' matches an entire (ZModZ7 i), and mod won't function on the custom type.
  mul a b = mod (a * b) 7       -- Based on discussion with monochrom@Freenode, equivalent to 
  --      = mod ( (ZModZ7 a') * (ZModZ7 b') )   
  -- Note that the values of a and b are still boxed.