Re: [Boost-users] [spirit] qi xml parser

Subject: Re: [Boost-users] [spirit] qi xml parser
From: Michael Powell (mwpowellhtx_at_[hidden])
Date: 2014-07-07 09:38:48

On Sun, Jul 6, 2014 at 11:13 PM, Gavin Lambert <gavinl_at_[hidden]> wrote:
> On 1/07/2014 08:14, Michael Powell wrote:
>>
>> So far so good, makes sense parsing names and so forth. However, how
>> do you handle element content? Which could either be a string, or zero
>> or more other elements (basically of the same rule as the enclosing
>> element rule).
>>
>> It would seem you need a terminus, the empty element tag. In such a
>> way that populates the parent (initial) element, and its children (of
>> the same element kind).
>
>
> Usually this sort of thing is resolved by using nodes -- a node is the basic
> base type of everything, and the entire tree can be treated as a homogenous
> tree of nodes.  But specific derived types of nodes vary in behaviour, eg.
> an element node represents a tag, an attribute node modifies its parent
> element, a text node can only contain basic text (no sub-nodes), a
> whitespace node can only contain whitespace (and can be discarded or
> reinserted unless it's a significant-whitespace node), etc.
>
> You could either use actual inheritance or you could make "node" a variant
> type over the various "subtypes".  Which one makes more sense may depend on
> how much implementation they end up wanting to have in common.

It's an interesting question, thank you for the insight. I get what
you mean, and yes, it is all about trade offs, between utility of the
domain model and developing a grammar that will help, and not hinder,
the domain model. The interesting questions are to do with how and
when to allow children (i.e. likely not in a comment node). It could
be that generally everything can be modeled as a tree, i.e. xobject
children of xobject parent, with appropriate specializations and/or
treatment of references, pointers, iterators, etc. Then the trade off
is trusting that the end user is responsible with his/her usage of
domain model.

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