1 files changed, 18 insertions, 0 deletions
diff --git a/lib/envelope.ml b/lib/envelope.ml
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+++ b/lib/envelope.ml
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+(** Envelope parser for tagged S-expressions.
+
+    Parses commands of the form (tag payload...) where the tag selects
+    which sublanguage handles the payload. The tag set is open — future
+    sublanguages (tcl, ppl, acl) just add new tags. *)
+
+type tagged = { tag : string; payload : Sexplib.Sexp.t }
+
+let parse (input : string) : (tagged, string) Result.t =
+  match Sexplib.Sexp.of_string input with
+  | Sexplib.Sexp.List (Sexplib.Sexp.Atom tag :: rest) ->
+    let payload = match rest with
+      | [single] -> single
+      | many -> Sexplib.Sexp.List many
+    in
+    Ok { tag; payload }
+  | _ -> Error "expected (tag payload...)"
+  | exception exn -> Error (Printexc.to_string exn)