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

The code that actually does the deed is, unfortunately, a bit on the hairy side. I believe that's largely because earlier, simpler code would sometimes lead to excessive compilation times. Twan van Laarhoven came up with the current code to address this.

The derived Functor instance always does the obvious thing. This is usually just fine, but occasionally misses opportunities. For example, suppose I write

data Pair a = Pair a a deriving Functor
data Digit a = One a | Two a a deriving Functor
data Queue a =
    Empty
  | Single a
  | Deep !(Digit a) (Queue (Pair a)) !(Digit a) deriving Functor

This will generate (in GHC 8.2)

instance Functor Queue where
  fmap ...
  x <$ Empty = Empty
  x <$ Single y = Single x
  x <$ Deep pr m sf = Deep (x <$ pr) (fmap (x <$) m) (x <$ sf)

It's possible to write that last case much better by hand:

  x <$ Deep pr m sf = Deep (x <$ pr) (Pair x x <$ m) (x <$ sf)

You can see the actual derived code using -ddump-deriv.