Use the toplevel pretty-printer from source code

toplevelpp.ml

(* ocaml -I `ocamlc -where`/compiler-libs ocamlcommon.cma toplevelpp.ml *)

module L = Longident;;

let print_value long_id value =
	let env = !Toploop.toplevel_env in
	let value_path, value_desc = Env.lookup_value long_id env in
	Toploop.print_value
		env
		(Obj.repr value)
		Format.std_formatter
		value_desc.Types.val_type;;

let x = 100;;

Format.pp_print_string Format.std_formatter "x = ";;
print_value (L.Lident "x") x;;
Format.pp_print_newline Format.std_formatter ();;

let y = "abc", "def";;

Format.pp_print_string Format.std_formatter "y = ";;
print_value (L.Lident "y") y;;
Format.pp_print_newline Format.std_formatter ();;

let z = 1234L;;

Format.pp_print_string Format.std_formatter "z = ";;
print_value (L.Lident "z") z;;
Format.pp_print_newline Format.std_formatter ();;

type complex = {re: float; im: float};;

let dummy_for_pp = {re = 0.0; im = 0.0};; (* dummy *)

let outer () =
	let local = {re = 3.14; im = 99.9} in
	Format.pp_print_string Format.std_formatter "z = ";
	(* print_value (L.Lident "local") local; *) (* <- error *)
	assert (dummy_for_pp == local || true); (* type checking *)
	print_value (L.Lident "dummy_for_pp") local; (* use type_expr of global value *)
	Format.pp_print_newline Format.std_formatter ()
in outer ();;

(* expected output:
x = 100
y = ("abc", "def")
z = 1234L
z = {re = 3.14; im = 99.9}
*)