RE: [xml-dev] Create a special purpose programming language, in XML, usi

[gkholman@CraneSoftwrights.com]

At 2013-10-18 13:50 +0000, David Lee wrote:
> To my thinking (perhaps why  I struggle with XSLT) ... an 
> <apply-templates> is essentially a GOTO
> with the current state (the current context)  as input and possibly 
> a 1 or more target states.
I disagree.

To me <xsl:apply-templates> isn't a "GOTO" it is a "Handle this" ... 
it is an event generator.  It generates an event for every node 
addressed by pushing that node at your stylesheet.

> and a <template> is a state description.
No, to me it is more like an event handler.  And you can have many 
event handlers that will handle the event based on the properties 
expected of the handler.  That is why there is a rule in XSLT that 
only one template rule can fire for each node pushed by the apply 
templates: there can be only one handler for every event (though that 
handler can trigger other events, even on the same node).

In a GOTO you know where you are going to.  In XSLT when you say 
"handle this", you are letting the processor decide which handler is 
going to handle it from the handlers you've prepared.

You don't really know which is going to handle it because your set of 
handlers might be overridden by an importing stylesheet that is 
supplying a different set of handlers.  Which is all good, because 
perhaps you need to change the behaviour and your set of handlers is 
read-only ... the importing stylesheet simply catches the node pushed 
at your stylesheet before the stylesheet finds it.

> The pattern matching ,to me, just adds to the difficulty of 
> understanding the GOTO's
Perhaps because they shouldn't be regarded as "GOTO"s in the first place.

>  ... as it makes
> understanding what the target of the GOTO harder to comprehend 
> (because it's really a function of the current state,
> which can be quite subtle).
>
> So what am I seeing wrong here ?  Maybe if I understood my 
> misconception I would do better at XLST.
Have a go at trying to picture it as event generation and event 
handling.  This is how I explain it to my students.

Check out the diagrams on pages 23, 247 and 251 of my XSLT book that 
is available for free download on a "try and buy" basis at 
http://www.CraneSoftwrights.com/training/#ptux ... if you decide not 
to pay for the book, please delete the copy that you download for free.

. . . . . . . . . . . Ken

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