[Boost-users] [spirit] Parsing a value multiple times

Subject: [Boost-users] [spirit] Parsing a value multiple times
From: Ravi (lists_ravi_at_[hidden])
Date: 2010-05-03 00:04:31

Hi,
  Is there a standard qi paradigm for parsing a value multiple times? Suppose 
a configuration file has three parameters "a", "b" and "c" each of which take 
integers. Here's how to parse them in arbitrary order:

struct abc { int a; int b; int c; };
..
// Inside the qi grammar with attribute abc
start = +(
  (lit("a") >> lit("=") >> int_[bind(&abc::a,_val)=_1] >> lit(";"))
 |(lit("b") >> lit("=") >> int_[bind(&abc::b,_val)=_1] >> lit(";"))
 |(lit("c") >> lit("=") >> int_[bind(&abc::c,_val)=_1] >> lit(";"))
  );

With the grammar above, the final value of "a" comes from the the last line in 
the configuration file of the form "a = 23;" overwriting all previous 
statements. This (later values overriding earlier ones) is a fundamental 
requirement for this application.

Is there a better way to write the rule above? Can semantic actions be avoided 
(perhaps by use of FUSION_ADAPT_STRUCT)?

As far as I can tell, using the "^" operator requires that the members of 
struct abc be boost::optionals. This grammar does not need optionals because 
the instance of struct abc that is passed to the parsing function is already 
initialized with sane defaults.

Second, composing grammars of this form does not seem to scale well. For 
example, suppose the configuration above is extended by one parameter, "d", 
for a different "composite" configuration file format. The easiest way to 
incorporate the new parameter is to copy and paste the old rule:

struct abcd : abc { int d; };
..
// Inside the qi grammar with attribute abcd
start = +(
  (lit("a") >> lit("=") >> int_[bind(&abcd::a,_val)=_1] >> lit(";"))
 |(lit("b") >> lit("=") >> int_[bind(&abcd::b,_val)=_1] >> lit(";"))
 |(lit("c") >> lit("=") >> int_[bind(&abcd::c,_val)=_1] >> lit(";"))
 |(lit("d") >> lit("=") >> int_[bind(&abcd::d,_val)=_1] >> lit(";"))
  );

The solution above is a clear DRY violation. Is there a solution that avoids 
copy-and-paste?

Ravi