data Bool = True | False

derive instance showBool :: Show Bool

This error shows up when you're attempting to derive an instance for a class for which compiler support does not exist. There is a fixed list of classes which can be derived by the compiler; for any class not on this list, however, you'll need to find another way of obtaining an instance.

• You will need to write an instance yourself.
• For many classes, you can use Generic to obtain the instances you want with less effort. For example you can use Data.Show.Generic to derive the Show instance using Generic.