[Haskell-cafe] Re: HaXml and the XHTML 1.0 Strict DTD

On 21/05/2008, at 5:44 PM, Malcolm Wallace wrote:

Peter Gammie wrote:

...

<!ELEMENT table (caption?, (col*|colgroup*), thead?, tfoot?, (tbody+|tr+))>

Using a slightly hacked HaXml v1.13.3, I get this from DtdToHaskell:

data Table = Table Table_Attrs (Maybe Caption) (OneOf2 [Col] [Colgroup]) (Maybe Thead) (Maybe Tfoot) (OneOf2 (List1 Tbody) (List1 Tr)) deriving (Eq,Show)

This looks entirely correct to me.

I realised that as soon as I sent it. :-)

...

My expectation is that we can have a <table> without a <col> or <colgroup> child.

Ah, yes I can see why that is permitted, but I guess HaXml's validator is not yet smart enough to be able to choose whether it has seen an empty list of <col> or an empty list of <colgroup>. :-)

Here is a suggested fix. Let me know if it works for you. In src/Text/XML/HaXml/Validate.hs, around line 220, use the following diff over the local defn of 'choice':

choice elem ns cps = -- return only those parses that don't give any errors [ rem | ([],rem) <- map (\cp-> checkCP elem (definite cp) ns) cps ] + ++ [ ns | all possEmpty cps ] where definite (TagName n Query) = TagName n None definite (Choice cps Query) = Choice cps None definite (Seq cps Query) = Seq cps None definite (TagName n Star) = TagName n Plus definite (Choice cps Star) = Choice cps Plus definite (Seq cps Star) = Seq cps Plus definite x = x + possEmpty (TagName _ mod) = mod `elem` [Query,Star] + possEmpty (Choice cps None) = all possEmpty cps + possEmpty (Choice _ mod) = mod `elem` [Query,Star] + possEmpty (Seq cps None) = all possEmpty cps + possEmpty (Seq _ mod) = mod `elem` [Query,Star]

Fantastic, thanks, that seems to work fine. A couple of nits: your use of `elem` refers to Prelude.elem, so I added the Prelude as a qualified import as P and changed those shadowed references to `P.elem`. I will try to send you a patch against 1.13.3 with all these little bits and pieces, when my project is finished. Can you lay out some kind of plan for HaXml? (is 1.13.x now dead, is 1.19.x stable, ...?) This would help for new-ish projects like mine.

Are there other places, apart from the validator, where a similar problem arises?

I do not know, I am merely using the DTD and HTML parsers, the CFilter combinators, the pretty printer and the validator. They all seem fine on a cursory check. (In general HaXml has been working quite well. Thanks for producing such a long-lived and well-thought-out library.) cheers peter