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Typeclasses

:t (+) 
---(+) :: Num a => a -> a -> a

When asking GHCI for the type of an operator like the plus-operator, you will notice that it is basically a function. Everything before the => sign is called the class constraint.

Creating a typeclass on our own:​

Creating instances manually​

We've seen before how to derive from some classes for our custom data, so we end up being able to use their functionality. Therefore, while data can automatically become instance of a class, we can also do it manually: 

data Options = Yes | No | Unsure

instance Eq Options where
  Yes == Yes = True
  No == No = True
  Unsure == Unsure = True
  _ == _ = False