There are many SVG elements, of which only a few are valid as the content of each other SVG elements. SvgDocumentElement defines the allowed subset for the SVG document.
I want to generate a "DList Char" for all those sub-elements and finally collapse them to one "DList Char" representing the whole SVG document. So it's a bit more complicated than your "Either" example
I suggest a monadic combinator approach. The grammar's the thing. Consider:
data View a = AtomicView a | NestViews (View a) (View a) (View a) | ConcatViews (View a) (View a) | Etc...
instance Monad View where return = AtomicView AtomicView v >>= f = f v (NestViews left middle right) >>= f = (ConcatViews (ConcatViews left middle) right) >>= f (ConcatViews left right) >>= f = ConcatVeiws (f left) (f right) -- Etc >>= f = whatever
These are "structural" nodes. Notice how >>= normalizes your document automagically. You would put your specific node types "in" Etc. Writing a renderer from something with this form is pretty straight forward. Enforcing constraints isn't too hard either. Neither is parsing. Just write a parser for each http://www.haskell.org/haskellwiki/Parsec http://legacy.cs.uu.nl/daan/download/parsec/parsec.html