How to define custom exception printers using Janestreet Core? - ocaml

Question

By default, a Failure exception is printed as such:
# print_endline (Printexc.to_string (Failure "uh\noh"));;
Failure("uh\noh")
For improved readability, we want to print the argument of Failure as is because we understand it should be human-readable. In the OCaml standard library, we would initialize an application with the following:
# Printexc.register_printer (function
| Failure s -> Some ("Failure: " ^ s)
| _ -> None
);;
The new behavior of Printexc.to_string would be:
# print_endline (Printexc.to_string (Failure "uh\noh"));;
Failure: uh
oh
Great. Now if we use the core_kernel library, first we can see that printing an exception is slightly different but not better to a human reader:
#require "core_kernel";;
# print_endline (Printexc.to_string (Failure "uh\noh"));;
(Failure "uh\
\noh")
Perhaps we can override this? Let's try.
# Printexc.register_printer (function
| Failure s -> Some ("Failure: " ^ s)
| _ -> None
);;
# print_endline (Printexc.to_string (Failure "uh\noh"));;
Failure: uh
oh
This works, but it's not using the printer that's part of Core_kernel. If we use it, we still get the same unreadable result:
# print_endline (Core_kernel.Exn.to_string (Failure "uh\noh"));;
(Failure "uh\
\noh")
And Core_kernel.Exn doesn't offer a register_printer function. So effectively, it looks like Core_kernel.Exn makes sure that we don't define custom exception printers. Is there another way or should we just not use Core_kernel.Exn then if we want to show human-readable error messages?
Edit: For context, our original problem is to print nested error messages nicely. For example, we would like to read something like the following:
Uncaught exception: Failure:
Uncaught exception in subprocess 1234: Failure:
something happened
trace line 1
trace line 2
trace line 1
trace line 2
trace line 3
where we use indentation for quoting and escaping, rather than double-quotes and backslash escape sequences.

Answer 1

Base.Exn (for which Core_kernel.Exn is an alias) prints errors as human readable s-expressions using Sexplib0 printer and converters. It is possible to add custom sexp converter for exceptions with Sexplib0.Exn_converter.add.
However, if you don't intend to print exceptions as s-expressions, I see indeed no reason to use the printer of Base.Exn.
EDIT: Since the problem seems to be the printing of S-expressions, one solution might be to use Base.Exn.sexp_of and then plug-in a custom S-expression printers that does not escape strings, nor print parenthesis:
let pp_sep ppf () = Format.fprintf ppf "# "
let pp_sexp ppf = function
| Atom s -> Format.pp_print_string ppf s
| List l ->
Format.fprintf ppf "#[<v 2> %a#]"
(Format.pp_print_list ~pp_sep pp_sexp) l
let to_string exn = Format.asprintf "%a" pp_sexp (Exn.sexp_of_t exn)