[plt-scheme] Re: Update SSAX/SXML for PLT?

From: John Clements (clements at brinckerhoff.org)
Date: Fri Jan 18 12:10:33 EST 2008

On Jan 18, 2008, at 2:37 AM, Dmitry Lizorkin wrote:

> Hello John!
> At our earlier convenience, I updated the SSAX/SXML packages on PLaneT
> an accordance with set-car!/set-cdr! removal:
> (require (planet "sxml.ss" ("lizorkin" "sxml.plt" 2 0)))
> The SXML package temporarily does not rely on HtmlPrag, as the latest
> HtmlPrag package on PLaneT still contains references to set-car! and
> set-cdr! itself. Please tell Neil Van Dyke to update his package
> accordingly, I will then uncomment the SXML-package functionality
> dependent on HtmlPrag.
> The bad news is that I had to remove the stx-engine from the package
> after all. The reason is that stx-engine relies on SXML parent
> pointers
> (http://okmij.org/ftp/Scheme/parent-pointers.txt
> subsection 'Parent "pointers" as SXML node annotations'), and IIUC one
> of reasons for removing set-car! and set-cdr! from PLT is to prevent
> the programmer from creating circular-style data structures.
> In the future, stx-engine can be modified in order not to rely on SXML
> parent pointer, but such a refactoring (together with the eval issue
> we discussed in the PLT mailing list recently) should better be
> performed via a complete stx-engine rewriting. I can be in charge of
> this, but I haven't got enough time for implementing this at the
> moment.

I'm delighted to hear this, and I know that others will be, as well.

In regards to the stx-engine: It doesn't bother me, as I don't  
believe that I depend on these features.  However, I want to make  
sure that you know that it is still perfectly possible to use mutable  
pairs in PLT Scheme... it's just that these are called 'mpairs', and  
created with 'mcons', etc.  You probably already knew this, but your  
message seemed to suggest that you believed it was no longer possible  
to create circular data structures in PLT Scheme, which is certainly  
not the case.

Again, many thanks from me and from other users of SXML/SSAX.

And again, forwarding without permission to plt-scheme, where others  
will be pleased to hear this and may have additional comments.


John Clements

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