Why isn't every type part of Eq in Haskell?

Question

Or rather, why isn\'t (==) usable on every data type? Why do we have to derive Eq ourseleves? In other languages, such as Python, C++, and surely o

Answer 1

How do you compare functions? Or existential types? Or MVars?

There are incomparable types.

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Edit: MVar is in Eq!

instance Eq (MVar a) where
        (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#

But it takes a magic primop to make it so.