I'd need some help to design a monadic datatype, I seem to have trouble wrapping my head around the idea, but I pretty definitely know what I want. Only the type checker seems to require some persuading in that matter.
What I want is some datatype Primitive that can be a monad. Functions of that are basically drawing primitives, and I want to be able to combine them and the combinations being primitives as well, much like the Parser type in the library parsec. But the actual drawing has to be done with an IO monad.
Incidentally, I also have a class Drawable a like this:
class Drawable a where
draw :: (MonadIO m) => Pos -> a -> m ()
And I want every Primitive of course to be drawable. Am thankful for any input.
I don't understand why you think this should be a monad. What's monadic about it?
You should design the API you want first, and incrementally think about how to abstract it.
Incidentally, Brent's diagrams library has a very nice compositional API that could provide some good ideas: http://byorgey.wordpress.com/2008/04/30/new-haskell-diagrams-library/