[Caml-list] Change in typing between OCaml 4.01.0 and 4.02.2...

[Caml-list] Change in typing between OCaml 4.01.0 and 4.02.2...

[Stéphane Glondu]

Hello,

I am debugging why monotone-viz doesn't compile with OCaml 4.02.2. I
have reduced the problem to the following:

$ cat query.mli
val make : #App.t -> unit
$ cat app.mli
class type status =
  object
    method pop : unit -> unit
  end

class type t =
  object
    method query : status -> unit
  end

val make : unit -> t
$ cat app.ml
class type status =
  object
    method pop : unit -> unit
  end

class type t =
  object
    method query : status -> unit
  end

class ctrl : t =
  object (self)
    method query status =
      Query.make self;
      status#pop ()
  end

let make () =
  new ctrl
$ ocamlc -c app.mli
$ ocamlc -c query.mli
$ ocamlc -c app.ml

With OCaml 4.02.2, I get the following error message:

  File "app.ml", line 15, characters 6-12:
  Error: This expression has type App.status
         It has no method pop

which looks wrong. With OCaml 4.01.0, there is no error.

Does anyone understand what is going on?


Cheers,

-- 
Stéphane

Re: [Caml-list] Change in typing between OCaml 4.01.0 and 4.02.2...

[Jeremy Yallop]

On 26 June 2015 at 19:07, Stéphane Glondu <steph@glondu.net> wrote:
> $ cat query.mli
> val make : #App.t -> unit

> $ cat app.ml
[...]
>       Query.make self;
[...]
> With OCaml 4.02.2, I get the following error message:
>
>   File "app.ml", line 15, characters 6-12:
>   Error: This expression has type App.status
>          It has no method pop
>
> which looks wrong. With OCaml 4.01.0, there is no error.
>
> Does anyone understand what is going on?

I think this is a result of tighter checks against self-references
when compiling modules:

   http://caml.inria.fr/mantis/view.php?id=6886

Re: [Caml-list] Change in typing between OCaml 4.01.0 and 4.02.2...

[Jacques Garrigue]

On 2015/06/27 04:15, Jeremy Yallop wrote:
> 
> On 26 June 2015 at 19:07, Stéphane Glondu <steph@glondu.net> wrote:
>> $ cat query.mli
>> val make : #App.t -> unit
> 
>> $ cat app.ml
> [...]
>>      Query.make self;
> [...]
>> With OCaml 4.02.2, I get the following error message:
>> 
>>  File "app.ml", line 15, characters 6-12:
>>  Error: This expression has type App.status
>>         It has no method pop
>> 
>> which looks wrong. With OCaml 4.01.0, there is no error.
>> 
>> Does anyone understand what is going on?
> 
> I think this is a result of tighter checks against self-references
> when compiling modules:
> 
>   http://caml.inria.fr/mantis/view.php?id=6886

Indeed, this seems to be the direct cause.
Note that the behavior of referring to oneself through an external
name was never properly defined (most often it would already fail
and even when succeeding it was referring to another definition).
The correct way to do this is either to duplicate the definition of
status in both app and query (removing the cyclic dependency),
or to define it in another file, which can be correctly referred to by both.

Yet, we should probably try to improve the error message,
which is indeed confusing. Not so easy, because this kind of
self-reference can be detected at very unexpected timings.
Can you add a comment to the above bug report?

Jacques Garrigue

Re: [Caml-list] Change in typing between OCaml 4.01.0 and 4.02.2...

[Stéphane Glondu]

Le 28/06/2015 11:44, Jacques Garrigue a écrit :
>> I think this is a result of tighter checks against self-references
>> when compiling modules:
>>
>>   http://caml.inria.fr/mantis/view.php?id=6886
> 
> Indeed, this seems to be the direct cause.
> Note that the behavior of referring to oneself through an external
> name was never properly defined (most often it would already fail
> and even when succeeding it was referring to another definition).
> The correct way to do this is either to duplicate the definition of
> status in both app and query (removing the cyclic dependency),
> or to define it in another file, which can be correctly referred to by both.

I did the latter in a patch that fixes the compilation of monotone-viz:

  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=790062#22

> Yet, we should probably try to improve the error message,
> which is indeed confusing. Not so easy, because this kind of
> self-reference can be detected at very unexpected timings.
> Can you add a comment to the above bug report?

Done.


Cheers,

-- 
Stéphane