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Re: port-with-print-state doesn't create a port? Or, when is a port not
From: |
Ludovic Courtès |
Subject: |
Re: port-with-print-state doesn't create a port? Or, when is a port not a port? :-) |
Date: |
Mon, 26 May 2014 18:26:24 +0200 |
User-agent: |
Gnus/5.130009 (Ma Gnus v0.9) Emacs/24.3 (gnu/linux) |
Mark H Weaver <address@hidden> skribis:
> address@hidden (Ludovic Courtès) writes:
>
>> Doug Evans <address@hidden> 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’.