I've been asked, and have previously considered, whether to add logic tags to TurboStan in order to facilitate "presentation logic". I'm still undecided whether this is a good idea or not.

Despite being firmly in the MVC camp, I am also a firm believer that there is such a thing as "presentation logic" that really belongs in the View (template).

For example, in a blog application, a template designer may want to specify that only 10 recent articles are displayed in a sidebar in one situation, and 20 in another. I don't expect a template designer to delve into the controller and add a one-off method to support this. I think the ability to specify things like this is a valid reason to have logic in a template.

On the other hand, once logic flow is added to templates, as it turns out, that is often the easiest place to do logic and it gets abused. Take a look at any moderately sophisticated Smarty template and you'll see what I mean. It quickly becomes a mess and it appears beyond the discipline of programmers and designers alike to avoid it.

So what's the solution? At the moment I've taken to allowing passing extra arguments to renderers in TurboStan, such as:

div ( id = 'sidebar',
      render = render.sequence,
      data = vars.model.Article.data.articles,
      selectby = { 'published': True, 'limit': 10 } )

Of course the controller has to accept these arguments, but that's easily doable.

The question is, is this enough? Is it inherently better than the alternatives? Should I add an "if" tag to TurboStan? You can already (ab)use list comprehensions and any other Python expression within TurboStan, but I wonder if it's wise to open this can of worms.

If you think I should add some sort of logic tag(s) ( I'm talking to you PacoPablo ), maybe a concrete example of something that can't easily be implemented without them would be convincing. An example that either a) cannot be done or b) is just too damn ugly to talk about would be best.

Personally I've found that my desire for such things has dropped pretty much proportionately to how much I've put aside what I'm used to and swallowed the Nevow pill of render and data callbacks, but I'm always willing to at least entertain other's ideas.