RFC: template engine [ was: Re: Implementing tables - advice wanted ]

Wed Jun 14 05:23:45 CEST 2006

Gaspar Chilingarov wrote:
> Post directed mainly to ke han, but all other list members are
> welcome -- You have mentioned html template engine for erlang
> - what kind of features do you like in there ?
>
> For current moment I got some simple engine which provides
> following templating commands -
[...]

I would personally avoid any imperative / non-declarative 
construct in a template language.
Tons of template languages exist for PHP, etc. so one should at 
least look at them and reuse ideas from them.
Many existing usable template engines / languages end up having 
the same characteristics / features:

- they usually apply the MVC pattern,

- in views, the pattern language allow to identify placeholders 
for values (strings) set by the controller (by the code), and 
allow to identify / markup blocks of text that are optional or 
that can be repeated, as controlled by the controller;

- some languages allow to recursively identify blocks inside 
blocks in views;

- the only information shared between a view (a template) and a 
controller (Erlang code) is: the names of value placeholders, 
and the names and structures (hierarchies) of 
optional/repeatable blocks.

The view language is usually completely declarative. If you need 
to do a loop to repeat a block, you should do it in the code (in 
the controller), not by defining a "FOREACH" construct in the 
template language (in the view).

A good, simple template language that can directly be used in an 
Erlang template engine in that of PHP Pear:
http://pear.php.net/manual/en/package.html.html-template-it.intro.php

For example, using this syntax, one view could be:

<html> 
 <table border> 
<!-- BEGIN row --> 
  <tr>
<!-- BEGIN cell -->  
   <td>
    {DATA}
   </td>
<!-- END cell -->  
  </tr>
<!-- END row --> 
 </table> 
</html>

The advantage of that syntax is that it allows most of the time 
to edit a template as HTML using any existing visual editor: 
nvu, dreamweaver, etc. This simplifies the design of templates 
by non-programmers. No loops, no programming logic in views.

Then, the template engine could allow a controller in Erlang to 
process a template like:

engine:parse_template("foo.tpl.html",
  [{block, row,
     [{block, cell, [{variable, DATA, "hello"}]},
      {block, cell, [{variable, DATA, "world"}]}]}])

which would return a string containing the whole parsed template.

PHP Pear also allows to maintain a cache of pre-parsed templates, 
to improve performance. This should be provided also by an 
Erlang template engine.

-- 
Romain LENGLET