How do I implement this transformation using generics?

The article describes a Haskell data type for integer lists that includes two constructors, `Cons1` and `Cons2`, where `Cons2` embeds a `DeeperList` (which itself wraps a `List`). The author wants to implement a function `incr` that increments each integer by a given value `k` plus the number of nested `DeeperList` levels surrounding that integer, and asks how to implement this using generic programming techniques from the `Data.Data` module, such as `gfoldl` or `gmapT`.

Let’s say I have a data type for integer lists. There are two Cons es: one that takes a normal List , and one that takes a DeeperList , which just embeds a List . data DeeperList = DeeperList List data List = Nil | Cons1 Int List | Cons2 Int DeeperList I’d like to write a function incr : incr :: Int - List - List such that incr k l increments each integer in l by k plus the number of DeeperList s that embeds that integer. For example: incr 3 $ Cons1 2 $ Cons2 4 $ DeeperList $ Cons2 0 $ DeeperList $ Cons1 5 Nil ------------------------------------------------------------------- -- represented more compactly as 2, 4, D 0, D 5 should be: Cons1 5 $ Cons2 7 $ DeeperList $ Cons2 4 $ DeeperList $ Cons1 10 Nil ------------------------------------------------------------------- -- 5, 7, D 4, D 10 I’d like to implement incr with generics, specifically with something like gfoldl , gmapT , etc., stuff from Data.Data . How do I do that?