What exactly does “deriving Functor” do?

Question

I\'m trying to figure out what exactly are the rules for deriving Functor in Haskell.

I\'ve seen message postings about it, and I\'ve seen test code abo

Answer 1

To use deriving Functor you must enable the DeriveFunctor language pragma and apply it to a polymorphic type which has a covariant final type variable---in other words, a type which admits a valid Functor instance. It'll then derive the "obvious" Functor instance.

There's been some concern in the past that the derived instance is not as efficient as a hand coded one is, though I cannot seem to find that material.

The algorithm itself was, as far as I could find, first proposed by Twan Van Laarhoven in 2007 and makes heavy use of Generic Haskell programming.