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Message-ID:  <06043efd-eaec-4484-4e80-1d072506d89c@morningstar2.co.uk>
Date:         Wed, 2 Sep 2020 11:21:46 +0100
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From:         Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: box manipulations, coffins and stacks
To:           LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
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Status: R

On 21/08/2020 11:04, Emmanuel Beffara wrote:
> Hello,
> 
> I have been reading this list for some time and I must say that I am
> impressed and enthusiastic on the recent work done on LaTeX3 and that it
> did renew my interest in TeX programming.
> 
> I am developing the ebproof package using expl3. The point of this
> package is to typeset proof trees made of inference rules, as people do
> in logic and computer science, which involves precise alignment
> operations and construction of boxes with precise dimensions. I just
> uploaded a bugfix release to CTAN because it broke with the new LaTeX
> release (this is my punishment for using a private function from the
> kernel although I knew I should never have done that) and it reactivated
> a few questions I had regarding the proper way of programming with
> boxes, so I thought I would finally ask here.
> 
> Firstly, the basic object that my package manipulates is a box equipped
> with extra information about the position of some elements inside, used
> for alignment. This is clearly the situation that coffins are designed
> to address and I would love to use coffins with extra poles for the
> operations I use. However I cannot do that currently (so I use boxes and
> property lists by hand) because there is apparently no way to extract
> information about the poles, a feature that is required for my use case.
> Essentially, I need to be able to compute the distance between two
> poles. More generally, I think getting the coordinates of a given handle
> would be a very useful thing to add to the public interface of coffins.
> Do you think it would make sense?
> 
> Secondly, the package works logically by manipulating a global stack of
> annotated boxes (this follows the tree structure of the material to be
> typeset and it is pleasant to work with). However, there is no natural
> way to implement stacks of structured data, the seq type only provides
> this functionality for balanced token lists. So my implementation uses
> (1) a global hbox to act as a stack of boxes, which works thanks to
> \box_set_to_last:N, and (2) a global seq that acts as a stack holding
> property lists, but that uses the fact that the prop type is actually
> implemented using tokens, and I am not sure this is good practice.
> Besides, if (when?) I move to using coffins, this will not work anymore.
> Do you think of a better approach to manipulate a stack of objects?
> The only robust approach I can think of is to allocate numbered
> variables on demand (like \g__stack_item_N) and use that as storage for
> the stack, but this unorthodox use of registers might cause problems I
> fail to think of right now.
> 
> Would someone have thoughts or insights on all this ?
> 
> Cheers,
> 

Hello Emmanuel,

Others are more qualified than me to comment on trees, etc. What I would 
note is that coffin poles are *relative* structures: if the coffin is 
resized, the absolute gap between them will alter.

Probably once the next LaTeX release is out, other team members will 
chip in.

Regards,

Joseph