sry for being a bit thick, but how would this code be used?
I'm unable to figure out the application yet. Could you give some examples how you use it?
Günther
So, the type (View view) -- ignoring class instances -- is basically isomorphic to this (slightly simpler) type: data View = EmptyView | TextView String | ConcatView View View | NestViews View View View | ... instance Monoid View where ... Now, consider the problem of "generic programming" on the simpler type: you quantify over the data constructors "generically", and in doing so you gain "traversals" for the type.[1] You gain the same things by turning View into (View view) -- a functor, a foldable functor, and so on. When it comes time to "render" a format for a View (for example, a bit of Html from Text.XHtml.Strict), I use some higher order functions I'm already familiar with. Something like renderXHtml :: (View view) -> Html renderXHtml (ConcatViews l r) = fold $ renderXHtml (ConcatViews l r) renderXHtml (NestViews l m r) = fold $ renderXHtml (NestViews l m r) renderXHtml (TextView string) = stringToHtml string renderXHtml (PageView v_title, v_heading, v_header, v_footer, v_contents) = (the_title << (renderXHtml v_title)) +++ -- (We assume v_title is a TextView String) (body << ( renderXHtml v_header ) +++ (render_page_contents v_contents v_heading) +++ (renderXHtml v_footer) ) where render_page_contents contents heading = undefined -- takes a View and uses the page's heading View -- so I guess we assume v_heading is a function -- into a TextView ... You could potentially use (>>=) for this, directly. And if you were using the simpler type, you could do the same thing with Uniplate, for example. It's going to construct an automorphism for you. [1] Actually, it's "the other way around". But the container/ contained adjunction makes them equivalent.