On Feb 20, 2010, at 10:25 AM, Heinrich Apfelmus wrote:
But isn't the line
renderXHtml (ConcatView l r) = fold $ renderXHtml (ConcatViews l r)
a type error? I'm assuming
Data.Foldable.fold :: (Foldable m, Monoid t) => m t -> t
being applied to the result type of renderXHtml which is Html and not of the form m t .
Yup, that's a type error. I mean to fold the View (in this case a ConcatView) into a monoid. I think I meant
foldMap renderXHtml (ConcatViews l r)
Your intention reminds me of the use of type variables to get functor-like behavior for free, like in
data RGB' a = RGB a a a -- auxiliary type constructor type RGB = RGB' Int -- what we're interested in
but I don't quite see what you're doing with the free monad here, Alexander?
As you noticed, I am seeking that functorial behavior in order to gain some genericity. bind and return do encode some logic about the nature of monadic adjunction, which I am relying on theoretically. I could have used a Functor instance just as easily, but I would have lost my "intention" of defining co-equalizers implicitly. (http://en.wikipedia.org/wiki/Beck%27s_monadicity_theorem ) Also, it was easier to write a monad instance than an Applicative instance, at least on my first try. ;-)