trace.simple: a basic span type as illustration

src/simple/dune

@@ -0,0 +1,6 @@
+
+(library
+  (name trace_simple)
+  (public_name trace.simple)
+  (synopsis "simple type for spans")
+  (libraries trace.core trace.util))

src/simple/simple_span.ml

@@ -0,0 +1,33 @@
+(** A simple span.
+
+    This is a concrete representation of spans that is convenient to manipulate.
+
+    @since NEXT_RELEASE *)
+
+open Trace_core
+
+type span_flavor =
+  [ `Sync
+  | `Async
+  ]
+
+type t = {
+  name: string;
+  __FUNCTION__: string option;
+  __FILE__: string;
+  __LINE__: int;
+  time_ns: int64;  (** Time the span was entered. *)
+  mutable time_exit_ns: int64;
+      (** Time the span was exited. Set at exit, [Int64.max_int] otherwise *)
+  tid: int;  (** Thread in which span was created *)
+  trace_id: int64;  (** For async spans *)
+  parent: parent;
+  flavor: span_flavor;
+  params: extension_parameter list;
+  mutable data: (string * Trace_core.user_data) list;
+      (** Modified by [add_data_to_span] *)
+}
+(** The type of spans used by all subscribers. *)
+
+type Trace_core.span +=
+  | Span_simple of t  (** How to turn a {!Simple_span.t} into a {!span}. *)