[Haskell-cafe] Re: I'm stuck in my thought experiment

[Levi Stephen]

>>> If I wanted to develop the widgets themselves separately from the 
>>> layout, I would probably do something like this:
>>>
>>> class Widget a where
>>>     render :: a -> Html
>>>     bbox :: a -> Size
>>>
>>> type Layout = forall a. Widget a => Widget a
>>>     | Rows Spacing [Layout]
>>>     | Columns Spacing [Layout]
>>>     | Grid Spacing [[Layout]]
>>>
>>> type Page = Page String Layout
>>>
>>> renderLayout :: Layout -> Html
>>>
>>> renderPage :: Page -> Html
>>
>> I'm unsure this gives what I'm after. I'm trying to have layouts 
>> consist of Widgets (e.g., header images, common menu), and as pages 
>> also consist of Widgets it seems like they can be modelled using a 
>> common type/construct.
> 
> Well if you want to abstract over the layout too, you can just add
> 
> instance Widget Layout where
>     render = renderLayout
>     bbox = ...
> 
> But just because you can, doesn't mean you should. I don't know the full 
> details of your design, but what do you gain by allowing the layout to 
> intermingle with the widgets? Is worth the extra complexity?
> 
> If you treat layout as "just another widget" then it becomes harder to 
> answer specific questions about the page layout because you have less 
> information in your tree.
> 

Layout might not actually be the right term. Page template might be better.

What I'm trying to gain is best described with an example.

* I have a template with a header image, and footer text.
* I create another template defined as the previous, but with a menu bar down
   the left.
* I create a page based on the previous with some text.

The gain comes from when I want to change the header image, or add a 
Login/Register box on all pages, I only edit the first template.

Levi