ppx: flatten tuples in poly constructors

We want to flatten the representation of polyvariants with tuple payload
and represent them as [TAG, ARG1, ARG2, ...] instead of [TAG, [ARG1, ARG2, ...]].

Fixes #24

CHANGES.md

@@ -4,6 +4,9 @@
   ([#13](https://github.com/melange-community/melange-json/pull/13))
 - PPX: Add `yojson` as runtime dep for the native version
   ([#15](https://github.com/melange-community/melange-json/pull/15))
+- PPX: change JSON representation of polyvariants, make it compatible with 
+  ppx_deriving_yojson and ppx_yojson_conv
+  ([#27](https://github.com/melange-community/melange-json/pull/27))
 
 ## 1.3.0 (2024-08-28)
 

ppx/tools/ppx_deriving_tools.ml

@@ -115,7 +115,12 @@ let register_combined ?deps name derivings =
 module Schema = struct
   let repr_row_field field =
     match field.prf_desc with
-    | Rtag (id, _, ts) -> `Rtag (id, ts)
+    | Rtag (id, _, []) -> `Rtag (id, [])
+    | Rtag (id, _, [ { ptyp_desc = Ptyp_tuple ts; _ } ]) -> `Rtag (id, ts)
+    | Rtag (id, _, [ t ]) -> `Rtag (id, [ t ])
+    | Rtag (_, _, _ :: _) ->
+        not_supported ~loc:field.prf_loc
+          "polyvariant constructor with more than one argument"
     | Rinherit { ptyp_desc = Ptyp_constr (id, ts); _ } ->
         `Rinherit (id, ts)
     | Rinherit _ ->