Re: [Caml-list] Printer for lists in toplevel is different when opening List

[Christophe Garion <tofgarion@runbox.com>]

On Wed, Feb 26 2020 at 07:36:48 PM, Florian Angeletti <octa@polychoron.fr> wrote:

> The printer for lists is indeed quite sensitive to the exact path of
> the type being printed.
>
> Any manipulation that muddles this path makes the printer revert to
> the generic sum type printer.
>
> Moreover, in the case of `List`, the list type is re-exported as
>
> type |'a| t  =|'a list|  =
> | []
> | (::) of 'a * 'a list
>
> Thus after opening the List module, the toplevel printer sees
>
> 1 :: (2 :: ...)
>
> as
>
> ( 1 :: (2 :: ... : 'a list) : 'a t)
>
> And since t is different that list, it prints the value as
>
> (::)(1, ...)
>
> then starting from the second element, we are back to 'a list and the
> pretty list printer.

OK, thanks for the explanation!

> This means that we can make the situation worse by re-exporting the
> type list as
>
> type |'a| t  =|'a list|  =
> | []
> | (::) of 'a * 'a t
>
> With this definition, we completely confuse the pretty printer for
> lists and are rewarded with:
>
> # [1;2];;
>
> - : int t = (::) (1, (::) (2, []))
>
> If you are interested, I have a small patch that makes the toplevel
> printer identify the list type more reliably:
> https://github.com/ocaml/ocaml/pull/9336 .

Thanks, as Nicolás asked me to open an issue, I will point to your PR.

Best,

Christophe

--
Christophe Garion
GPG: 1982 15B2 64AC 3C34 532D  BF19 6CD6 246C 62DA 5A7F