Is it possible to particularly use the default implementation of a function?

Question

My code works. I just would like to know if it is possible to just add something in an instance declaration to be checked first before using the default implementation, instead of having to copy the code. An example:

class (Eq a, Ord a, Show a, Num a) => Fibo a where
  fib :: a -> a
  fib n
    | n == 0 = 0
    | n == 1 = 1
    | otherwise = fib (n-1) + fib (n-2)

instance Fibo Integer where
  fib n
    | n < 0 = -1
    | n == 0 = 0
    | n == 1 = 1
    | otherwise = fib (n-1) + fib (n-2)

But can it also be written in a way where I dont have to re-implement the whole function? Something like:

instance Fibo Integer where
  fib n
    | n < 0 = -1
    | otherwise = default

Or is there any other way to do without using the same code in two places?

The Solution is NOT to change the default implementation!

Answer 1

Just put the common code to a helper function:

foo::(Eq a, Ord a, Show a, Num a) =>a -> a
foo n
    | n == 0 = 0
    | n == 1 = 1
    | otherwise = foo (n-1) + foo (n-2)

class (Eq a, Ord a, Show a, Num a) => Fibo a where
  fib :: a -> a
  fib = foo

instance Fibo Integer where
  fib n
    | n < 0 = -1
    | otherwise = foo n