How to properly instantiate classes in Haskell?

Question

Trying to create a base class from which I can derive different types. What's wrong with the following?

class (Eq a) => MyClass a 

data Alpha = Alpha
instance MyClass Alpha where
    Alpha == Alpha = True

I get the error:

test.hs:5:10: `==' is not a (visible) method of class `MyClass'
Failed, modules loaded: none.

Answer 1

You have to make Alpha an instance of Eq explicitly. This will work:

data Alpha = Alpha
instance Eq Alpha where
    Alpha == Alpha = True
instance MyClass Alpha