Re: [Boost-users] [spirit]how to ignore the sequence of the rules?

From: Joel de Guzman (joel_at_[hidden])
Date: 2007-09-11 18:49:15

james.jones_at_[hidden] wrote:
>> From: moosefly <moosefly_at_[hidden]> For example, I want to parse the
>> following text:
>> 
>> <input type=hidden value=1>
>> 
>> I have defined 2 rules match the "type" field and the "value" field, but
>> the sequence of 2 rules is uncertain, that means "<input value=1 
>> type=hidden>" is also legal,so I can't define the rules just like:
>> 
>> rule_type >> !space_p >> rule_value
>> 
>> The following works at this simple condition:
>> 
>> ( (rule_type >> !space_p >> rule_value) | (rule_value >> !space_p
>>>> rule_type ) )
>> But if there are many out-of-order fields need to parser, how can I ignore
>> the sequence of the rules?
> 
> What you seem to want to do (correct me if I'm wrong) is to have some set of
> N attributes, each of which may or may not be present, but can appear only
> once, in arbitrary order, and you want to check this in the parser. This is
> not a great idea, because it leads to a combinatorial explosion of states.
> There's really no way out of this (strictly within the parser).

FWIW, Spirit-2 now has this kind of parser, called the permutation parser.
Yes, it is tricky to do in Spirit-1, but not impossible.

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net