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From Daniel Fagerstrom <dani...@nada.kth.se>
Subject Re: xml languages
Date Sat, 22 Jan 2005 13:11:13 GMT
BURGHARD √Čric wrote:
>>I'm working on refactoring JXTG so that it will be easier to support and
>>develop. I'm aware about that you have access the request object etc
> 
> Great ! Don't know if i can help (time), but if you have a road map or
> something like that, i would be happy to contribute.

I will write a roadmap. All contributions are welcome, a simple but 
important contribution is to start using the refactored JXTG instead of 
the original and report or even better send patches for the bugs.

>>differently depending on if you are using flow or not and also that the
>>eval tag is less well designed. What more weird behaviours are you
>>thinking of?
> 
> Variable nature and scope (kind of non-mutable that you can overwrite ?),
> but someone could tell me this a feature :-). Perhaps i've miss something,
> but look what i need to do to retrieve a value incremented inside a forEach
> loop:
> 
> <jx:set var="globalvars" value="${java.util.HashMap(5)}"/>
> <jx:set var="dummy" value="${globalvars.put('a_count', 0)}"/>
> ...
> <jx:set var="a_count" value="${globalvars.a_count}"/>
> <jx:forEach begin="1" end="3" varStatus="i">
>    ...
>    <jx:if test="#{$node/@path != ''}">
>       ...
>       <jx:set var="a_count" value="${a_count+1}"/>
>       <jx:set var="dummy" value="${globalvars.put('a_count', a_count)}"/>
>   </jx:if>
> </jx:forEach>
> 
> <jx:out>this is my count: ${globalvars.a_count}</jx:out>
> 
> It would be easiest (and efficient) to do
> <jx:set var="a_count" value="0"/>
> <jx:forEach begin="1" end="3" varStatus="i">
>    ...
>    <jx:if test="#{$node/@path != ''}">
>       ...
>       <jx:set var="a_count" value="${a_count+1}"/>
> </jx:forEach>
> 
> <jx:out>this is my count: ${a_count}</jx:out>
> 
> the fact that "set" make always a "new define" is quite annoying at first
> glance (functional programming ?).

The expression context is implemented as a stack of maps and the put 
instruction on the context is a put on the top map. ForEach is 
implemented in such way that the loop is performed in a local map that 
is pushed on stack before the loop and poped afterwards. The set 
instruction will do a put on the top map and will disapear afterwards 
because of that.

Ok, that explains the behaviour, the question is what we should do about 
the, admitingly, weird behaviour.

One possibilty would be to decide that we want JXTG to be more like a 
functional language. In that case we should deprecate jx:set and 
introduce a jx:let instead that just gives a local name for an 
expression and that gives an exception if you try to set the "variable" 
to a new value. I think I would prefer this behaviour as I would prefer 
having a template language without side effects.

Another possibility is to let set asign the value to the first variable 
binding with the same name that it finds when the stack is searched, 
instead of creating a new binding at the top of the stack if the name 
doesn't exist on the top of the stack. If we go for this we need two 
constructions one that declare and possibly gives an intial value to a 
variable in the current context and one that binds the uppermost 
occurance of the name, a jx:declare and a jx:set e.g.

Also we would need to change the API and behaviour of ExpressionContext 
so that we both have a declare and a set method. This should probably be 
done anyway.

WDYT?

/Daniel


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