Question

I declare my data type as an instance of class Num and implement only method sum (+). The other methods like (*), negate, abs, fromInteger, etc. I don’t implement. The reason is I don’t need these methods for my data type.

I compile the programme, and it’s ok except some uncomfortable warnings such as:

Warning: No explicit method nor default method for `*'
...

How can I avoid these warnings, or do I need to implement the above methods? Actually, I don’t want to implement them.

Answer 1

You can silence these warnings with a command line flag (in GHC)

ghc -fno-warn-missing-methods YourFile.hs

But if you only want to implement addition, then it is probably not appropriate to make your type an instance of Num. If you want just to have a nice operator for an operation on the type, you can define your own, you can even re-use (+) if you

import Prelude hiding ((+))
import qualified Prelude  -- so that (+) can be still used in qualified form