[web-devel] Re: A separation of concerns

Bardur Arantsson spam at scientician.net
Mon Apr 5 05:49:50 EDT 2010

On 2010-04-03 23:28, John Bender wrote:
> Simon,
> 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>
>     <ul>
>      <todo:list>
>        <li>
>         <todo:check><input type="checkbox"/></todo:check>
>         <todo:priority>
>           <select><option>1</option></select>
>         </todo:priority>
>         <todo:desc>To Do</todo:desc>
>        </li>
>      </todo:list>
>     </ul>
>   </div>
> </lift:TD.list>
> private def doList(reDraw: () =>  JsCmd)(html: NodeSeq): NodeSeq =
>   toShow.
>   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

Disclaimer: I've only done a very simple (and incomplete) application in 
Liftweb. YMMV.

Anyway, regardless of what you do there has to be _some_ connection 
between the view and the controller (backend) and in my experience, the 
"View first" approach of Liftweb does a very good job here.

A major advantage is that the view is 100% pure XHTML/HTML so that it 
can be edited in any XML editor. It also contains placeholder data so 
that most of the layout/design can be done independently of having the 
actual application running. (That's the theory, anyway. I haven't done 
this on any sort of large scale, so again YMMV.)

The only approach I can think of which would do away with having a 
binding mechanism would be to generate all the front end HTML/XHTML/JS 
completely from Haskell. This is great for developers, but not so good 
for the "web designer" crowd.


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