Haskell fmap functor

Question

I have a problem with functors over queue based on designated algebraic datastructures.

data DQueue a = Empty | Enqueue a (DQueue a)
    deriving (Eq, Show, Read)


instance Functor DQueue
  where
    fmap f (Enqueue x xs) = Enqueue (f x) $ fmap f xs

instance Foldable DQueue
  where
    foldr = error "not done"

sample1 :: DQueue Int
sample1 =  Enqueue 5 $ Enqueue 7 $ Enqueue 9 Empty

and result should be like that:

fmap (+1) sample1 ~?= Enqueue 6 (Enqueue 8 (Enqueue 10 Empty))
foldr (+) 0 sample1 ~?= 24

fmap seems to be logicaly correct but I get an error: Non-exhaustive patterns in function fmap

Thank you in advance.

Answer 1

Your Functor instance definition is non-exhaustive because it doesn't handle all possible type constructors of DQueue. Namely, it is missing a pattern matching Empty. You need to define how to handle fmap when your value is Empty.