# How do I implement this transformation using generics?

> Source: <https://discourse.haskell.org/t/how-do-i-implement-this-transformation-using-generics/14153#post_1>
> Published: 2026-05-23 20:54:40+00:00

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?