Haskell no instance for (Eq Digit) arising from a use of == In the expression: x == One

Question

data Digit = Zero | One
convBNum :: [Digit] -> Int
convBNum [] = 0
convBNum (x:xs)
  | x == One = 2^length xs + convBNum xs
  | otherwise = convBNum xs

This is the code of a simple function that converts a binary number into an int; when I compile it, it gives me this error:

no instance for (Eq Digit) arising from a use of == 
In the expression: x == One

If I understood well, also reading other questions, the problem is that x can't be anytype, but I couldn't understand how to fix this and how to use Eq constraint.

Answer 1

Add a deriving clause

data Digit = Zero | One deriving (Eq)

or do not use == , and use pattern matching instead:

convBNum (One:xs) = 2^length xs + convBNum xs
convBNum (_  :xs) =               convBNum xs

Answer 2

the problem is that x can't be anytype

In your code x has type Digit and that's exactly the type it should have. That's not the problem.

The problem is that the == operator is defined in the Eq type class. This means that it can only be used on types that are instances of that type class, which your Digit type is not. To make it one, you could supply your own definition of == for Digit like this:

instance Eq Digit where
    Zero == Zero = True
    One == One = True
    _ == _ = False

However definitions for some type classes (like Eq , Ord , Show and Read ) can be automatically defined for custom data types by using the deriving keyword, which is usually preferable. So you could write:

data Digit = Zero | One deriving Eq

And this will generate the above Eq instance automatically.

Note that for your use case, using nested pattern matching would be more idiomatic than using == :

convBNum :: [Digit] -> Int
convBNum [] = 0
convBNum (One : xs) = 2^length xs + convBNum xs
convBNum (Zero : xs) = convBNum xs

This way you don't even need the Eq instance, but it makes sense to have it anyway as Digit is a type that really should support == .

On an unrelated note, you should probably also either derive or manually define an instance for Show , so that you can print digits and display them in GHCi.