Blocking bug: some graders fail with `Error: Ctype.Unification_trace.Unify(_)`

[erikmd]

Related project scope(s) (ex: client, CSS, grading, etc...) : grading
Related user(s): @erikmd

Bug description

I noticed this week a regression:
some graders of my lab sessions (relying on and extending code from Test_lib, so involving direct calls to Introspection) worked fine with 0.12 but now fail noisily with 0.13.x with the following error:

Learnocaml v.0.13.1 running.
Updating app at ./www
Exercise definition error while testing your solution:
File "_none_", line 1:
                                                       Error: Ctype.Unification_trace.Unify(_)


grader outcomes:
val print_html : 'a -> 'b = <fun>
module Code :
(…)

To Reproduce

I was able to craft a minimal working example:

• Add just the following code in an existing test.ml:

module type tARITH_TEACHER = sig
  type t
  val zero : t
  val one : t
  val add : t -> t -> t
  val mul : t -> t -> t
  val opp : t -> t
  val of_int : int -> t
  val to_string : t -> string
end

(* dummy but sufficient example to reproduce the failure *)
let foo0 () = Introspection.get_value "INT" [%ty: (module tARITH_TEACHER)]

• then you'll get a failure with 0.13.1.

@AltGr, would you have an idea to isolate/fix the issue?

I hope this could be fixed or workarounded (so that the grader behaves the same as with 0.12).

Current configuration

• OS: GNU/Linux
• Browser: Firefox
• Version: learn-ocaml 0.13.1

[erikmd]

I skimmed the compiler-libs code, so it seems the error comes from a call to the unify function:

https://github.com/ocaml/ocaml/blob/46c947827ec2f6d6da7fe5e195ae5dda1d2ad0c5/typing/ctype.ml#L3114-L3121

But I can't see where the exception Ctype.Unification_trace.Unify would be un-caught in learn-ocaml's code after the 4.12 migration… hoping @AltGr will have some hints

[lsylvestre]

Hi @erikmd,
Hi @AltGr,

Here is another example:

let x = 10 ;;
let v = Introspection.get_value "x" [%ty: int] ;;

It produces the following trace:

Exercise definition error while testing your solution:
File "_none_", line 1:
                                                       Error: Ctype.Unification_trace.Unify(_)
...
module Test_lib : Test_lib.S
module Report = Learnocaml_report
val x : int = 10
demo                       [FAILED]

Workaround

Like this, it works:

module I = Introspection

let x = 10 ;;
let v = I.get_value "x" [%ty: int] ;;

module Introspection = Introspection

module type tARITH_TEACHER = sig
  type t
  val zero : t
  val one : t
  val add : t -> t -> t
  val mul : t -> t -> t
  val opp : t -> t
  val of_int : int -> t
  val to_string : t -> string
end

let foo0 () = Introspection.get_value "INT" [%ty: (module tARITH_TEACHER)]

Strange, isn't it?

So, to fix (temporarily) the problem, you can add the following code:

      handle_error (internal_error [%i"while preparing the tests"]) @@
      Toploop_ext.use_string ~print_outcome ~ppf_answer
        "module Introspection = Introspection" ;

to the file: https://github.com/ocaml-sf/learn-ocaml/blob/master/src/grader/grading.ml#L143

have a good day,

Loïc

[erikmd]

Thanks a lot, @lsylvestre ! ✨

I'll merge your workaround now to be able to use ocamlsf/learn-ocaml:master;
(and we'll be able to refine the fix later on if need be - I added a comment in the code)