When should we put parenthesis after deriving?

Question

I see two expressions

data BinTree a = BTNil | BTNode a (BinTree a) (BinTree a) deriving Show

data Day = Sunday | Monday | Tuesday | Wednesday
         | Thursday | Friday | Saturday         deriving (Enum)

I am confusing when I should use parenthesis after deriving. I know we should use parenthesis and commas where there are more than one classes.

Answer 1

In every sensible use case, there's no distinction between the two.

... deriving (A)
... deriving  A

are perfectly equivalent. As you've already correctly pointed out, the parentheses are necessary if you have multiple classes to derive from. Personally, I always include the parentheses, simply for the sake of consistency. But it's entirely a style choice, and as long as you're consistent, it doesn't really matter.

---

For the sake of absolute completeness, there is technically one case where it's necessary. If for some reason you have a typeclass which is an operator name, you will need to supply the parentheses to make it work.

{-# LANGUAGE TypeOperators, MultiParamTypeClasses,
    GeneralizedNewtypeDeriving #-}

class (:+) a

-- newtype Foo a = Foo a deriving   :+   -- Definitely a syntax error
-- newtype Foo a = Foo a deriving  (:+)  -- Confuses the parser (error)
newtype Foo a    = Foo a deriving ((:+)) -- Works