ocamlPackages.ocamlformat-rpc-lib: downgrade for ocaml <= 4.12

required for ocaml-lsp

pkgs/development/ocaml-modules/ocamlformat-rpc-lib/default.nix

@@ -1,12 +1,24 @@
-{ lib, fetchurl, buildDunePackage, csexp, sexplib0 }:
+{ lib, fetchurl, buildDunePackage, ocaml, csexp, sexplib0 }:
+
+# for compat with ocaml-lsp
+let source =
+  if lib.versionAtLeast ocaml.version "4.13"
+  then {
+    version = "0.21.0";
+    sha256 = "sha256-KhgX9rxYH/DM6fCqloe4l7AnJuKrdXSe6Y1XY3BXMy0=";
+  } else {
+    version = "0.20.0";
+    sha256 = "sha256-JtmNCgwjbCyUE4bWqdH5Nc2YSit+rekwS43DcviIfgk=";
+  };
+in
 
 buildDunePackage rec {
   pname = "ocamlformat-rpc-lib";
-  version = "0.21.0";
+  inherit (source) version;
 
   src = fetchurl {
     url = "https://github.com/ocaml-ppx/ocamlformat/releases/download/${version}/ocamlformat-${version}.tbz";
-    sha256 = "sha256-KhgX9rxYH/DM6fCqloe4l7AnJuKrdXSe6Y1XY3BXMy0=";
+    inherit (source) sha256;
   };
 
   minimumOCamlVersion = "4.08";