As I'm not familiar with rank-N types, the type signature of gfoldl is a troublesome for me:
gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b)
-> (forall g. g -> c g)
-> a
-> c a
The only functions I can think of are \xs y -> ($y) <$> xs and pure, respectively.
Other functions such as gunfold and gmapT have similar problems. So what are notable examples of nontrivial uses of them?
For the gmapT case the mkT function is defined for this purpose in the original paper.
mkT :: (Typeable a, Typeable b ) => (b -> b) -> a -> a
mkT f = fromMaybe id (cast f)
For example to increment all int fields in the A, you can write something like
data A = A {f :: Int, s :: Int} deriving (Data, Typeable)
ex = gmapT (mkT inc) (A 2 3) where
inc :: Int -> Int
inc = (+1)
To make it clearer the ex function can be written like this too:
ex2 = gmapT f (A 2 3) where
f :: (Data a ) => a -> a
f a = case cast a of
Nothing -> a
(Just (b :: Int)) -> fromJust $ cast (b + 1)