How can I make my type an instance of Arbitrary?

I have the following data and function

data Foo = A | B deriving (Show)

foolist :: Maybe Foo -> [Foo]
foolist Nothing  = [A]
foolist (Just x) = [x]

prop_foolist x = (length (foolist x)) == 1

when running quickCheck prop_foolist, ghc tells me that Foo needs to be an instance of Arbitrary.

No instance for (Arbitrary Foo) arising from a use of  ‘quickCheck’
In the expression: quickCheck prop_foolist
In an equation for ‘it’: it = quickCheck prop_foolist

I tried data Foo = A | B deriving (Show, Arbitrary), but this results in

Can't make a derived instance of ‘Arbitrary Foo’:
  ‘Arbitrary’ is not a derivable class
  Try enabling DeriveAnyClass
In the data declaration for ‘Foo’

However, I can't figure out how to enble DeriveAnyClass. I just wanted to use quickcheck with my simple function! The possible values of x is Nothing, Just A and Just B. Surely this should be possible to test?