TODO

#========================================================================
#
# TODO
#
# DESCRIPTION
#   TODO list for the Template Toolkit version 2.20, containing
#   known bugs, limitations, planned enhancements, long term visions 
#   and a few whacky ideas.  Development on TT2 has effectively 
#   ceased for everything but bug fixes.  All new features and general
#   enhancements are being saved for TT3.
#
# AUTHOR
#   Andy Wardley   <abw@wardley.org>
#
#========================================================================


#------------------------------------------------------------------------
# Miscellaneous
#------------------------------------------------------------------------

* The 'eval' filter leaks memory, as reported by Colin Johnson.  The
  filter subroutine created contains a reference to the context and then
  gets cached in the FILTER_CACHE item of the context.  Hey presto -
  circular references.  The reset() method should probably clear the
  FILTER_CACHE.  Also need to check the plugins cache for similar
  problems.  UPDATE: this may now have been fixed.

* The handling of the DELIMITER parameter could be improved.  At the 
  moments it's hardcoded and hacked to Do The Right Thing for Win32
  but I'd prefer it to Do The Good Thing.

* If you use 'ttree' with a COMPILE_EXT or COMPILE_DIR option then
  templates in the 'lib' directories will be compiled, but those in
  the src directories will not.  This is because ttree does a chdir()
  to the src directory and processes files as './myfile'.  TT doesn't
  compile RELATIVE files by default.

* No recursion checking is performed for BLOCKs, only
  Template::Document instances.  This is probably the way it will stay
  (unless anyone shouts loudly enough) but it should be documented
  anyway.  STOP PRESS: I had an idea that bare BLOCK subs should be
  blessed into Template::Document class to allow $template->process()
  to be called regardless.  Template::Document methods would need to
  test $self for CODE/HASH and Do The Right Thing.  This would then
  allow recursion testing for BLOCKs as well as Template::Document
  objects.

* It would be nice if there was an option so that the files generated
  under the COMPILE_DIR are relative to the INCLUDE_PATH and not absolute.
  This could cause potential conflicts (e.g. if INCLUDE_PATH changes
  between sessions and the same files in different INCLUDE_PATH dirs
  maps to the samed compiled version) but is convenient for those times
  when you know that's not going to be a problem.

* Craig Barratt notes, in fixing the problem with NEXT not working 
  inside SWITCH (see Changes v2.04):

    By the way, I came across another arcane bug:

	NEXT FOREACH k = [1];

    is legal syntax but is an infinite loop, since $_[0]->{ INFOR } in
    Parser.yp is not set when the NEXT is parsed, so it generates a
    plain "next;" rather than calling $factor->next().  I don't see an
    easy, clean fix.


#------------------------------------------------------------------------
# Documentation
#------------------------------------------------------------------------

* Extend the FAQ.


#------------------------------------------------------------------------
# Directives
#------------------------------------------------------------------------

* A 'FOR', like 'FOREACH' but without using an iterator.  You wouldn't get 
  the 'loop' reference to test 'first', 'last', etc., against, but it would
  be faster for those cases when you didn't need it.  This will likely 
  be implemented as a facility feature (see later).

* PRINT should be defined as a new directive, doing what the print() 
  method of Template::View currently does (the Right Thing).

    [% PRINT node %]    ===    [% tt.view.print(node) %]

  NOTE TO SELF: this is a Very Good Idea [tm].  PRINT becomes the way to 
  display a data structure (e.g. hash, list, XML element, MyThingy, database
  record, etc.) in an "intelligent" fashion.  Implemented underneath via 
  the current default VIEW.

* ARGS.  There may be a requirement for reusable template components
  to define what variables they plan to use.  This would allow some
  optimisation and also possibly help to avoid global variable clashes.
  Would also be a useful "comment" directive for human readers and maybe
  also help in debugging (WARNING: expected 'title' argument).

    [% ARGS title		# no default
	    bgcol='#ffffff'	# default value
    %]


#------------------------------------------------------------------------
# Parser
#------------------------------------------------------------------------

* Lists don't accept arbitrary expressions as elements, although 
  function arguments now do.  So you can do this: [% foo(bar + 1) %],
  but you can't do this: [% foo = [bar + 1] %].  This has been fixed in 
  the v3 parser.  

* The parser isn't as intelligent as it could be about blocks of template
  code commented out en masse.  The pre-scanner find the first terminating 
  END_TAG after an opening tag, regardless of it being on a 
  commented line or not.
  e.g.
    [%#
      # 
      #  [% INCLUDE blah %] <- directive ends here
      #  foo                <- this gets printed
    %]

* Craig Barratt reports the following:

  I looked at Parse.yp to see how hard it would be to push FILTER
  evaluation down into the expr rule, so that you could put filters
  inside expressions (eg: using repeat() just like &quot;x&quot; in
  perl).  More about that later.

  In browsing through Parser.yp I noticed several issues:

  - The operator precedence is very different to perl, C etc.
    For example, these expressions evaluate differently in
    TT2 versus perl, C etc:

      + "1 || 0 && 0" evaluates to 0 in TT2 and 1 in perl or C.  
        TT2 parses it as (1||0) && 0; in perl and C && is higher
        precedence than ||.

      + "1 + !0 + 1" evaluates to 1 in TT2 and 3 in perl or C.
        TT2 parses it as 1 + !(0 + 1); in perl and C ! is higher
        precedence than +.

      + Many other expressions parse incorrectly, but the effect
        is benign since most rules return flat text that perl
        correctly re-parses.  Eg, 2 * 3 + 4 is incorrectly parsed
        as (2 * (3 + 4)), but happily just the string "2 * 3 + 4"
        is compiled by perl, which correctly evaluates it as
        (2 * 3) + 4.

  - There is no unary minus and the NUMBER token is signed. So you can
    write "x = -2;" but not "x = -y;". Moreover, "x = 1 -1;" is a syntax
    error (since "1 -1" returns just two tokens NUMBER, NUMBER). (As a
    workaround you can rewrite these as "x = 0-y;" and "x = 1 - 1".)

  - You cannot have expressions in lists ([..]) and function arguments.

  I have modified the Parser.pm (to make NUMBER unsigned) and modified
  Grammar.pm.skel and Parser.yp to fix most of these issues (improved
  operator precedence, unary minus and plus), and also to allow
  expressions in a few more places (eg: range).  But the last item
  has me stuck.

  The parse rules for lists and function arguments make COMMA optional,
  so you can equivalently write [1 2 3 4] or [1,,,,,2 3 4] or [1,2,3,4].
  This makes it very difficult to make each term an expression, because
  the resulting grammar has many ambiguities.  For example, is [1 -1]
  two elements [1, -1] or a single element [0]?  One partial solution is
  to move the bracketed expression rule '(' expr ')' to the term rule,
  allowing expressions to be included via parens.  But there are also
  ambiguities, eg: does [foo (1+1)] have 2 elements or is it a function
  call to foo?

  Without allowing expressions in lists or function arguments, the unary
  minus change I've made means that the NUMBER token is unsigned, so with
  my changes you cannot write [-1, 2, 3].  Not a good thing.

  One solution is to change the grammar so that COMMAs are required in
  lists and arguments, but that would break several test cases and
  probably break lots of old templates.  But this might be the only
  way to produce a grammar that is a lot more similar to perl.

  Another solution is to ignore these issues altogether and use temporary
  variables to precompute expressions that you need in lists or function
  arguments, or use explicit lvalue assignments, eg:

    foo(x + 2);             becomes    temp = x + 2;
                                       foo(temp);

  or

    List = [x+1,x+2,x+4];   becomes    List   = [];
                                       List.0 = x+1;
                                       List.1 = x+2;
                                       List.2 = x+4;

  Both of these look ugly to me.

  Back to the FILTER issues. Ultimately I'd like to be able to embed filters
  as low precedence operators in expressions, and write:

    List = [
        "foo" | repeat(10),
        "bar" | repeat(10)
    ];

  but I doubt there is a non-ambiguous upward compatible grammar that
  supports this.

  Comments?


#------------------------------------------------------------------------
# Plugins
#------------------------------------------------------------------------

* We need a way to easily enable/disable certain plugins.  This should
  be addressed by facility provision.  Probably something for v3.

* The Template::Plugin DBI iterator first/last() methods don't behave 
  the same as list first/last().  Randal also reports that get_all()
  doesn't work as it should - may be a conflict in code/docs?  Again,
  this is a problem to solve in TT3.

* PLUGINS could accept a reference to an object which is used as a 
  singleton factory for a plugin.  (NOTE: 2.01 includes PLUGIN_FACTORY
  to implement this, but currently undocumented because it's likely to
  change).

* A more general solution for XML (e.g. DOM, XPath, etc) would be for
  TT to support a PerlSAX handler which generates the appropriate
  callbacks to the view.  This should make it possible to easily
  display XML content from XML::DOM, XML::XPath, or any other SAX
  compliant source.

  Something like this:

    # define a view
    [% VIEW my_view 
         prefix="my/xml/dom/path/" ;
       END
    %]

    # get some XML
    [% USE dom = XML.DOM %]
    [% doc = dom.parser(my.files.xmldata) %]
    
    # ask the view to print the data
    [% my_view.print(doc) %]

  The view print() method will call the relevant 2SAX method on the 
  XML node, passing a SAX2TTView handler to make the relevant calls 
  back to the view to display parts of the XML data model as SAX events
  are received.


#------------------------------------------------------------------------
# Views
#------------------------------------------------------------------------

The current implementation is there to get me (and anybody else who's
interested) using it and trying to identify the problems, requirements
and general issues involved.  I've got a better idea now about what a
VIEW should be in notional terms, but I'm still not quite sure about
the syntax and API.

General thoughts:

* A view defines a set of templates.  Things like prefix, suffix, 
  default, etc., can be specified to customise template selection.
  In this sense, it is like a custom provider of those templates.
  It implements the template() method to fetch a template according
  to those rules.

* It is also a custom processor of those templates.  It implements the 
  process() method.  In this sense, it is like a custom context.

* It also implements dispatch logic to apply the right template to the
  right kind of data.  It does this via the print() method.  It may 
  have all kinds of custom dispatch logic.

* A view takes responsiblity for things template related as opposed 
  to anything data related (stash) or application logic related
  (plugins, runtime code, etc).  It is the user interface facility
  within the engine.