[ZPT] Proposed changes to the TAL specification

[Fred Drake]

This is a proposal I've added to the ZPT wiki as:

  http://zope.org/Wikis/DevSite/Projects/ZPT/RemoveInsaneConstructs

I've cleaned up the structured text markup a bit for this email, but
there really
aren't any interesting differences.  I'd like to get relevant feedback soon, as
I'd like to produce the new specification and use it for Zope X3 3.1.

---------------------------------------------------------------
Proposal

  The TAL 1.4 specification provides a number of loopholes for confusion.
  These appear to be issues where specific combinations of TAL statements
  weren't considered when the specification was written.  As TAL has been
  more broadly used, and especially as it's seen adoption in  Zope 3, some
  less carefully crafted corners of the specification have been identified.

  This proposal suggests some specific improvements that should be made to
  the specification, which can be folded in to create a new revision.  It is
  expected that at least the Zope 3 TAL interpreter can be improved as a result.

  Specific Changes

    - 'tal:replace' and 'tal:content' can insert new markup structures into the
      output using the 'structure' keyword.  The TAL 1.4 specification does not
      take a stance on whether this structure should be parsed and interpreted
      in any way; implementations are *allowed* to ignore embedded TAL
      constructs, but are permitted to interpret them.

      In the case when interpretation is permitted, attribute
replacements specified
      using 'tal:attributes' are made to the first element found in
the replacement.
      It is not clear that this behavior is useful, but it is very fragile.

      The Zope 3 implementation attempts to support the insertion of attributes,
      and does so by honoring all TAL within the replacement, but in the absence
      of 'tal:attributes' does not respect TAL embedded in the
replacement structure.

      PETAL (the Perl TAL/METAL implementation) does not support interpretation
      of TAL in the replacement structure.  I don't know about other
implementations.

      The specification should be modified to disallow the interpretation of
      replacement structure.

    - If TAL in a replacement structure can never be interpreted,
there is no value
      to ever combine 'tal:replace' and 'tal:attributes'.  This should
be explicitly
      identified as an error in the template.  'tal:content' and
'tal:attributes' still
      makes sense.

References

  "structure" and TAL interpretation
  (http://mail.zope.org/pipermail/zpt/2004-July/thread.html#5441) was a thread
  on the ZPT mailing list where it was generally agreed that parsing a
  replacement structure was undesirable.


  -Fred

-- 
Fred L. Drake, Jr.    <fdrake at gmail.com>
Zope Corporation