Add Sexp.kind

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

src/stdune/sexp.ml

@@ -508,6 +508,16 @@ module Of_sexp = struct
     (x, [])
 
   let record t = enter (fields t)
+
+  type kind =
+    | Values of Loc.t * string option
+    | Fields of Loc.t * string option
+
+  let kind : type k. k context -> k -> kind * k
+    = fun ctx state ->
+      match ctx with
+      | Values (loc, cstr, _) -> (Values (loc, cstr), state)
+      | Fields (loc, cstr, _) -> (Fields (loc, cstr), state)
 end
 
 module type Sexpable = sig

src/stdune/sexp.mli

@@ -118,6 +118,14 @@ module Of_sexp : sig
       S-expressions to parse *)
   val eos : (bool, _) parser
 
+  (** What is currently being parsed. The second argument is the atom
+      at the beginnig of the list when inside a [sum ...] or [field
+      ...]. *)
+  type kind =
+    | Values of Loc.t * string option
+    | Fields of Loc.t * string option
+  val kind : (kind, _) parser
+
   (** [repeat t] use [t] to consume all remaning elements of the input
       until the end of sequence is reached. *)
   val repeat : 'a t -> 'a list t