feat(Logger): add register_printer

src/Logger.ml

@@ -72,4 +72,12 @@ struct
       ~init_backtrace:(get_backtrace())
       ~emit:(fun d -> emit (m d))
       ~fatal:(fun d -> fatal (m d))
+
+  let register_printer f =
+    Printexc.register_printer @@ function
+    | Effect.Unhandled (Emit diag) -> f (`Emit diag)
+    | Fatal diag -> f (`Fatal diag)
+    | _ -> None
+
+  let () = register_printer @@ fun _ -> Some "Unhandled asai effect/exception; use Asai.Logger.run"
 end

src/LoggerSigs.ml

@@ -133,4 +133,14 @@ sig
       @param emit The interceptor of non-fatal diagnostics. The default value is {!val:emit}.
       @param fatal The interceptor of fatal diagnostics. The default value is {!val:fatal}. *)
   val try_with : ?emit:(Code.t Diagnostic.t -> unit) -> ?fatal:(Code.t Diagnostic.t -> 'a) -> (unit -> 'a) -> 'a
+
+  val register_printer : ([ `Emit of Code.t Diagnostic.t | `Fatal of Code.t Diagnostic.t ] -> string option) -> unit
+  (** [register_printer f] registers a printer [p] via {!val:Printexc.register_printer} to convert unhandled internal effects and exceptions into strings for the OCaml runtime system to display them. The functor {!module:Logger.Make} always registers a simple printer to suggest using {!val:run}, but you can register new ones to override it. The return type of the printer [p] should return [Some s] where [s] is the resulting string, or [None] if it chooses not to convert a particular effect or exception. The registered printers are tried in reverse order until one of them returns [Some s] for some [s]; that is, the last registered printer is tried first. Note that this function is a wrapper of {!val:Printexc.register_printer} and all the registered printers (via this function or {!val:Printexc.register_printer}) are put into the same list.
+
+      The input type of the printer [p] is a variant representation of all internal effects and exceptions used in this module:
+      - [`Emit diag] corresponds to the effect triggered by {!val:emit}; and
+      - [`Fatal diag] corresponds to the exception triggered by {!val:fatal}.
+
+      Note: {!val:Diagnostic.string_of_text} can be handy for converting a message into a string.
+  *)
 end