printing the exception in Internal_error causes a failure

[agarwal]

$ cat stack.ml 
class ['a] stack init = object
  val mutable v = init

  method pop = 
    match v with
    | hd :: tl -> 
      v <- tl;
      Some hd
    | [] -> None

  method push hd = 
    v <- hd :: v
end ;;

$ ./app.native stack.ml 
((pid 85452) (thread_id 0)
 ((human_readable 2015-05-18T15:57:48-0400)
  (int63_ns_since_epoch 1431979068026404000))
 "unhandled exception in Async scheduler"
 ("unhandled exception"
  ((lib/monitor.ml.Error_
    ((exn "Assert_failure lib/conv.ml:487:15")
     (backtrace
      ("Raised at file \"lib/conv.ml\", line 487, characters 15-27"
       "Called from file \"lib/conv.ml\", line 215, characters 26-43"
       "Called from file \"lib/conv.ml\", line 229, characters 17-44"
       "Called from file \"lib/conv.ml\", line 234, characters 8-27"
       "Called from file \"lib/exn.ml\", line 38, characters 50-65"
       "Called from file \"list.ml\", line 73, characters 12-15"
       "Called from file \"list.ml\", line 73, characters 12-15"
       "Called from file \"lib/core_list.ml\", line 328, characters 16-30"
       "Called from file \"lib/result.ml\", line 56, characters 16-19"
       "Called from file \"lib/deferred.ml\", line 15, characters 64-67"
       "Called from file \"lib/job_queue.ml\", line 164, characters 6-47" ""))
     (monitor
      (((name main) (here ()) (id 1) (has_seen_error true)
        (is_detached false) (kill_index 0))))))
   (Pid 85452))))

The outcome of this script ends up being an Uneval of an Internal_error, and the printing of such a value is the source of the above error. If we comment out the exception printer here, then the problem goes away:

$ ./app.native stack.ml 
(* part 0 *)
# class ['a] stack init = object
  val mutable v = init

  method pop = 
    match v with
    | hd :: tl -> 
      v <- tl;
      Some hd
    | [] -> None
  method push hd = 
    v <- hd :: v
end ;;
oloop-top: sending the phrase eval outcome failed because: output_value: functional value

A second bug is that the specific error seems wrong. The expected outcome is:

Error: Some type variables are unbound in this type:
         class ['a] stack :
           'b list ->
           object
             val mutable v : 'b list
             method pop : 'b option
             method push : 'b -> unit
           end
       The method pop has type 'b option where 'b is unbound

[Chris00]

Rough guess: there may be an exception that should be added to the serialization mechanism of oloop-top that is transformed into an `Internal_error because of that. That exception may carry closures (e.g. an environment) which causes the other problems.

[Chris00]

The unhandled case is Typeclass.Error(_, _, _).