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Re: [Fwd: Re: Language is not markup and markup is not langu

Subject: Re: [Fwd: Re: Language is not markup and markup is not language.]
From: James Clark <jjc@xxxxxxxxxx>
Date: Tue, 11 May 1999 22:28:16 +0700
james clark turing
Paul Prescod wrote:
> David Carlisle wrote:
> >
> > Is it clear that it isn't complete anyway?
> I'm not up to proving that XSL isn't Turing complete but I will give some
> hints about why I think that it isn't:
> > You have recursive calls, and can pass state via template parameters
> > what else do you need?
> You can pass state but can you work with the state? Remember that the
> Turing machines has not only a concept of "current state" but also its
> tape. What would we use in XSL to emulate the tape? The obvious choice is
> a string, but how do you index to a particular location in the string? The
> string manipulation functions don't seem up to the job.

You can use a string that separates the representation of each location
by some delimiter (say a /); then you can use basic arithmetic,
recursion and substring-before/substring-after to index to a particular

It seems fairly clear that the language is now Turing complete.

> Compared to early drafts, however, it is incredibly flexible. Only time
> will tell what optimizations are feasible but it is safe to say that some
> optimization opportunities have probably been lost. With the early drafts
> statically type checking and optimizing seemed almost doable but by now I
> am very skeptical. Maybe we need a smaller, more optimizable XSL subset
> for some applications....

It's possible now to write XSLT stylesheets that will be very hard to
optimize; but that's always been possible; for example XSL has always
had recursive macros. I don't think the addition of new capabilities to
XSLT need prevent optimization. If you had some stylesheet that could be
expressed using an earlier WD, then it can still be expressed in a
similar way in the current WD and it is equally susceptible to
optimization. If an optimization would be possible except for some new
feature in XSLT, you don't have to completely give up on that
optimization, rather you can analyze the stylesheet to determine whether
it makes use of that feature and then only apply the optimization if the
stylesheet does not make use of the feature.  


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