Re: expected_function

On 30/04/2016 1:04 AM, Stefan Dyulgerov wrote:
> Hello,
>
> does any one know why the expect_function has a member first?
>
> I was trying to see why i do not get on_error<fail> code and it seems, that
> the expect_function class has a logic that prevents some of the exceptions.
> Most notably there is a member called first, which eats all the exceptions
> in the beginning.
>
> I am trying to parse the word
>
> imports,
>
>     qi::rule < iterator, std::string(), ascii::space_type >
> m_identifier;
>     m_identifier = raw[ lexeme[(alpha | '_') > *( alpha | digit | '_')] ];
>
> imports is parsed
> 1imports is not parsed, however this code is not called:
> boost::throw_exception(Exception(first, last, component.what(context)));
>
> If i comment the logic seems to work fine. I see this is connected with some
> multipass parsing, which i do not use.

The expression a > b, is specified as:

Match a followed by b. If a fails, no-match. If b fails, throw an expectation_failure<Iter>

Hence, only b matters. If a does not match, then it is simply a parse
error, not an expectation failure.

In X3, you can use the expect directive instead:

   expect[a >> b]

HTH

reference: 
http://www.boost.org/doc/libs/1_60_0/libs/spirit/doc/html/spirit/qi/reference/operator/expect.html

Regards,
-- 
Joel de Guzman
http://www.ciere.com
http://boost-spirit.com
http://www.cycfi.com/


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