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Functor class (base)
import Data.Functor

The Functor class is defined like this:

class Functor f where
  fmap :: (a -> b) -> f a -> f b

It is available in the Prelude, but defined in Data.Functor.

All instances of Functor should obey:

fmap id      = id
fmap (p . q) = (fmap p) . (fmap q)

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