# deriving implementation in OCaml

The best code is code that does not exist, and in that regard, Haskell has great support for deriving implementation (that became even better with `deriving via`).

``````{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE KindSignatures, PolyKinds#-}

import Data.Kind (Type)

data NTree (a :: Type) =
NLeaf a
| NNode (NTree (a,a))
deriving (Eq, Ord, Read, Show, Functor, Foldable, Traversable)
``````

As far as I can tell, the same in OCaml requires a bit of manual plumbing

``````
type 'a n_tree = NLeaf of 'a | NNode of ('a * 'a) n_tree (* [@@deriving map] fails *)

let rec map_ntree : 'a 'b. 'a n_tree -> ('a -> 'b) -> 'b n_tree =
fun t f ->
match t with
| NLeaf x -> NLeaf (f x)
| NNode p -> NNode (map_ntree p (fun (l, r) -> (f l, f r)))

``````

What's the status of these derivations in OCaml?

Is there a better way to supply automatically the corresponding proof trees as of now?

Would it be hard to make some similar more powerful `deriving` extension?

Solution

• There are a few ppx derivers available in opam, try `opam search ppx`. As an example, you can use ppx_deriving, e.g., in the OCaml top-level,

``````# #use "topfind";;
# #require "ppx_deriving.std";;
# type 'a n_tree = NLeaf of 'a | NNode of 'a * 'a n_tree
[@@deriving show, eq, ord, iter, fold, map];;
type 'a n_tree = NLeaf of 'a | NNode of 'a * 'a n_tree
val pp_n_tree :
(Ppx_deriving_runtime.Format.formatter -> 'a -> Ppx_deriving_runtime.unit) ->
Ppx_deriving_runtime.Format.formatter ->
'a n_tree -> Ppx_deriving_runtime.unit = <fun>
val show_n_tree :
(Ppx_deriving_runtime.Format.formatter -> 'a -> Ppx_deriving_runtime.unit) ->
'a n_tree -> Ppx_deriving_runtime.string = <fun>
val equal_n_tree :
('a -> 'a -> Ppx_deriving_runtime.bool) ->
'a n_tree -> 'a n_tree -> Ppx_deriving_runtime.bool = <fun>
val compare_n_tree :
('a -> 'a -> Ppx_deriving_runtime.int) ->
'a n_tree -> 'a n_tree -> Ppx_deriving_runtime.int = <fun>
val iter_n_tree : ('a -> unit) -> 'a n_tree -> unit = <fun>
val fold_n_tree : ('a -> 'b -> 'a) -> 'a -> 'b n_tree -> 'a = <fun>
val map_n_tree : ('a -> 'b) -> 'a n_tree -> 'b n_tree = <fun>
``````