utop and printing constructors

Question

When loading up code in utop (or dune utop) and printing any data constructor like Bin.Stop with #show Bin.Stop;; I get a crash.

utop # #show Bin.Stop;;
>> Fatal error: Ident.rename Bin.nat
Fatal error: exception Misc.Fatal_error

~ via  v4.12.0 took 4m41s
❯

While ocaml does not crash. Is this a bug or is there an obvious reason why that would be illegal ?

(* SOTypeLevelNat.ml *)
module Bin : sig
  type stop = Qasdpaokdsap

  type 'a zero = Sasddkasdo

  type 'a one = Zoldjsfoij

  type _ nat =
    | Stop : 'stop nat
    | Times2      : 'n nat -> 'n zero nat
    | Times2Plus1 : 'n nat -> 'n one nat
end = struct
  type stop = Qasdpaokdsap

  type 'a zero = Sasddkasdo

  type 'a one = Zoldjsfoij

  type _ nat =
    | Stop : 'stop nat
    | Times2      : 'n nat -> 'n zero nat
    | Times2Plus1 : 'n nat -> 'n one nat
end

utop # #use  "SOTypeLevelNat.ml";;
module Peano :
  sig
    type 'a s = S
    type z = Z
    type 'n nat = Zero : 'a nat | Succ : 'n nat -> 'n s nat
  end
module Bin :
  sig
    type stop = Qasdpaokdsap
    type 'a zero = Sasddkasdo
    type 'a one = Zoldjsfoij
    type _ nat =
        Stop : 'stop nat
      | Times2 : 'n nat -> 'n zero nat
      | Times2Plus1 : 'n nat -> 'n one nat
  end

printing a type, say, the nat works just fine

utop # #show Bin.nat;;
type nonrec _ nat =
    Stop : 'stop Bin.nat
  | Times2 : 'n Bin.nat -> 'n Bin.zero Bin.nat
  | Times2Plus1 : 'n Bin.nat -> 'n Bin.one Bin.nat

Answer 1

Fatal errors with toplevel directives that happen in utop and not ocaml are bugs in utop.

Indeed, utop partially reimplements the driver logic of the OCaml REPL and it can happen that utop's code ends up slightly out-of-date with the underlying OCaml's compiler libraries.

Please report issues like this at: https://github.com/ocaml-community/utop/issues