That's tested in the actual tests.

Also, in Haskell, type checking and being able to compile is usually good enough as "example tests".

Suggestion

I don't see how a Num instance would improve the Kata.

The examples do not necessarily show Haskell code, my intention was to convey a bit of the theory through some code. Additionally it makes the Show instance easier to define, to focus on what's really important about this Kata.

I don't think this is an issue, but thanks for your feedback anyway!

Yes, you are right!