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

[Malcolm Wallace]

Peter Gammie <peteg42 at gmail.com> 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.

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

Regards,
    Malcolm