Re: port-with-print-state doesn't create a port? Or, when is a port not a port? :-)

[ludo@gnu.org (Ludovic Courtès)]

Mark H Weaver <mhw@netris.org> skribis:

> ludo@gnu.org (Ludovic Courtès) writes:
>
>> Doug Evans <xdje42@gmail.com> skribis:
>>
>>> The problem can be succinctly represented by the following:
>>>
>>> scheme@(guile-user)> (port? (port-with-print-state (current-output-port)))
>>> $3 = #f
>>
>> I think the short answer is that it’s a very old API that’s essentially
>> unused internally.  [...]
> [...]
>> I think the problem it was trying to solve has been solved differently
>> (by explicitly passing the print state in the print.c code, for
>> instance), and can easily be solved differently.
>
> In order to implement SRFI-38 properly and efficiently, I think we need
> to somehow pass the print state to user-defined structure printers.
> Among other things, the print state includes a map from the set of
> objects that are currently being printed (i.e. the ancestors of the
> current object) to the associated datum label.
>
> Aside from proper SRFI-38 support, the print state is also used to
> specify parameters such as maximum-depth for printing abbreviated
> structures, used for example by the backtrace printer.

Good points.

> As distasteful as this 'port-with-print-state' concept may be, I'm not
> aware of a better solution.  Fluids aren't quite right, because a
> structure printer might cause I/O to happen on another port.
>
> Another alternative would be to explicitly pass the print state to
> structure printers, and then provide versions of 'write' and 'display'
> that accept a separate print state argument, but we'd still need to
> handle all the existing struct printers that don't know about this.
>
> Yet another option would be to move the print state into the port
> itself.  It might be worth considering, although it seems a bit unclean.

Maybe the port alist you added a few months ago could be used to
implement that actually, no?

Thanks,
Ludo’.