[Zpt] TALES

[Evan Simpson]

I added comments to Todd's comments on TALES.  I have a few issues for
discussion.

First, I suggest adding a way to handle existence testing:

Add expression type "exists", which takes a path expression and returns a
boolean value indicating whether the path successfully resolves::

    <span z:condition="exists:template/annotation"
z:replace="template/annotation">

Alternatively, make path expressions that fail to resolve evaluate to
singleton value "Undefined" instead of raising an exception.  "Undefined"
would be false, so we could write::

    <span z:define="ann template/annotation" z:condition="ann"
z:replace="ann">

Next, how should path resolution in Python expressions look?  I think that
we have agreed to make all template variables available to Python
expressions directly, so that we can simply write expression like "python: x
+ y".  What about paths, though?  The alternatives seem to be::

   <span z:define="x here/to/there" z:condition="python: x == 1">
   or
   <span z:condition="python: here.restrictedTraverse('to/there') == 1">

Is this acceptable?

Cheers,

Evan @ digicool