feat: add Modifier.S.register_printer

src/Modifier.ml

@@ -83,4 +83,11 @@ struct
     Effect.Deep.try_with f () @@ handler ~not_found ~shadow ~hook
 
   let try_with ?(not_found=Perform.not_found) ?(shadow=Perform.shadow) ?(hook=Perform.hook) f = run ~not_found ~shadow ~hook f
+
+  let register_printer f =
+    Printexc.register_printer @@ function
+    | Effect.Unhandled (NotFound {context; prefix}) -> f (`NotFound (context, prefix))
+    | Effect.Unhandled (Shadow {context; path; former; latter}) -> f (`Shadow (context, path, former, latter))
+    | Effect.Unhandled (Hook {context; prefix; hook; input}) -> f (`Hook (context, prefix, hook, input))
+    | _ -> None
 end

src/ModifierSigs.ml

@@ -80,6 +80,10 @@ sig
         try_with ~shadow @@ fun () ->
         (* even more code *)
       ]}
-
   *)
+
+  val register_printer : ([ `NotFound of context option * Trie.bwd_path | `Shadow of context option * Trie.bwd_path * (data * tag) * (data * tag) | `Hook of context option * Trie.bwd_path * hook * (data, tag) Trie.t ] -> string option) -> unit
+  (** [register_printer f] registers a printer [f] via {!val:Printexc.register_printer} to turn unhandled effects into strings for the runtime system to display the error of unhandled effects. The functor {!module:Modifier.Make} will register a simple printer to suggest using {!val:run} to capture effects, but you can register new printers to override it. The registered printers are executed in reverse order; that is, the last registered printer is executed first. The return type of the printer [f] is [string option], where [None] means the printer passes on the (unhandled) effect and lets the runtime system try the next printer or (if no more printers are available) use the default system message.
+
+      The input type of [f] is a variant representation of all internal effects used in this module. They correspond to the three effect triggers in {!module:Perform}. More precisely, [`NotFound (ctx, path)] corresponds to the effect triggered by [Perform.not_found ctx path], [`Shadow (ctx, path, x, y)] corresponds to [Perform.shadow ctx path x y], and then [`Hook (ctx, path, hook, t)] corresponds to [Perform.hook ctx path hook t]. *)
 end