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
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