diff --git a/lib/recorder.ml b/lib/recorder.ml
new file mode 100644
index 0000000..d583917
--- /dev/null
+++ b/lib/recorder.ml
@@ -0,0 +1,22 @@
+open! Core
+open Syntax
+open Concrete_domains
+(* open Interp_effects *)
+
+type event =
+  | Evt_update_st of (Path.t * Label.t * (value * Job_q.t))
+  | Evt_set_dec of (Path.t * decision)
+  | Evt_enq_eff of (Path.t * clos)
+  | Evt_alloc_pt of Path.t
+
+(** TODO: *)
+let record = function
+  | Evt_update_st (path, label, (v, q)) -> ignore (path, label, v, q)
+  | Evt_set_dec (path, dec) -> ignore (path, dec)
+  | Evt_enq_eff (path, clos) -> ignore (path, clos)
+  | Evt_alloc_pt path -> ignore path
+
+type diagnostics
+(** TODO *)
+
+let diagnostics = ref (failwith "not implemented")
diff --git a/lib/recorder.mli b/lib/recorder.mli
new file mode 100644
index 0000000..763e940
--- /dev/null
+++ b/lib/recorder.mli
@@ -0,0 +1,15 @@
+open! Core
+open Syntax
+open Concrete_domains
+
+type event =
+  | Evt_update_st of (Path.t * Label.t * (value * Job_q.t))
+  | Evt_set_dec of (Path.t * decision)
+  | Evt_enq_eff of (Path.t * clos)
+  | Evt_alloc_pt of Path.t
+
+val record : event -> unit
+
+type diagnostics
+
+val diagnostics : diagnostics ref