[web-devel] A separation of concerns

John Bender john.m.bender at gmail.com
Sat Apr 3 17:28:17 EDT 2010


While I don't have any hands on experience with Lift's approach to
templating it looks a lot like asp.net databinding/templates and from my
experience with that toolset it ends up being a cumbersome solution to the
problem of separating controller and view. I don't necessarily think mixing
code into your views is the "one true way" either, but what you sacrifice in
controller code clarity seems to outweigh any gains from separating the two
so strictly.

As an example from
http://liftweb.net/docs/getting_started/mod_master.html#x1-130002.7 :

<lift:TD.list all_id="all_todos">
 <div id="all_todos">
   <div>Exclude done <todo:exclude/></div>
       <todo:check><input type="checkbox"/></todo:check>
       <todo:desc>To Do</todo:desc>

private def doList(reDraw: () => JsCmd)(html: NodeSeq): NodeSeq =
 flatMap(td =>
   bind("todo", html,
      "check" -> ajaxCheckbox(td.done,
               v => {td.done(v).save; reDraw()}),
      "priority" ->
      ajaxSelect(ToDo.priorityList, Full(td.priority.toString),
               v => {td.priority(v.toInt).save; reDraw()}),
      "desc" -> desc(td, reDraw)

In the above you'll notice that the bind has direct reference to elements in
the template, so I think its fair to say you've moved some of your template
mixing from view to the controller. From the docs

"This method calls toShow to get the list of to-do items to show. Using the
flatMap method, we iterate over each to-do item and binding the item to the
incoming html."

In addition I think the result is something thats harder for people to grok.
When reading loops and control flow constructs from a programming language
in a template its immediately familiar. When binding data into a template,
at the very least, the code you need to reference exists in two separate
places but interacts very very intimately.

Again, I'm not saying that the current practice of writing code into the
template should be the final solution. You could make the case that Haskell
is better at parsing and handling text for whatever purpose (databinding or
otherwise) than almost any other language and that, as a result, it would be
better equipped to handle controller side manipulation of xhtml. In addition
this might allow you to better leverage the type system.

Personally I think if your going to be mucking around with html/xhtml a bit
of haskell won't make it much worse :D


To contribute something positive here, I would really like to see HAML
become the goto template system of choice for haskell web dev if only
because the syntax meshes well (indentation, terseness) and there's an
enormous sea of web developers already familiar with its use and best
practice. One less thing to worry about when learning webdev in haskell.



Disclaimer: I am an experienced web developer but I'm relatively new to
Haskell. If there is some aspect of the language, unknown to me, that makes
the Lift templating/databinding system much more effective feel free to

On Sat, Apr 3, 2010 at 11:36 AM, Simon Hengel <simon.hengel at wiktory.org>wrote:

> > My point is: we should not try to build one big framework. Instead, I
> > propose that we build a set of smaller libraries that each do one
> > thing very well. Some of these libraries might be designed to work
> > together.
> I absolutely agree with that.
> > I personally think MVC is a perfect fit for the web: model code
> > handles data storage, view code handles HTML/JSON/XML and the
> > controller coordinates between these.
> Have you read about the "View First" approach of Lift[1]? While I do not
> like the monolithic approach of Lift, I really like that specific
> aspect. I'd not narrow my view on "Rails-Style MVC" right now.
> Cheers,
> Simon
> [1] http://wiki.liftweb.net/index.php?title=Lift_View_First
> _______________________________________________
> web-devel mailing list
> web-devel at haskell.org
> http://www.haskell.org/mailman/listinfo/web-devel
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