[Boost-users] [proto] Handling String Terminals

Subject: [Boost-users] [proto] Handling String Terminals
From: David A. Greene (greened_at_[hidden])
Date: 2010-02-07 17:05:42

If I have a grammar like this:

typedef boost::proto::literal<std::string>::type StringTerminal;

typedef or_<
  StringTerminal,
  ...
> Rule;

and I do this:

  checkMatch<Rule>(boost::proto::lit(std::string("str")));
  checkMatch<Rule>(std::string("str"));
  checkMatch<Rule>("str");

where checkMatch is a wrapper around matches<>, the first
line is the only one that passes.  This is presumably because
the string needs to be "protoized" and to match StringTerminal
it has to be cast to std::string explicitly.

This is rather ugly for constructs like this:

term["str"]  => term[lit(string("str"))]

I would really rather not have to do the lit(string()) thing.

Is there a good strategy to avoid this clutter?  I could predefine
literals like this:

StringTerminal strlit = {{"str"}}.

term[strlit]

but that's a bit burdensome on the user.

Would the same massaging be needed for, say,

term + "strlit"

Perhaps I can reformulate the grammar to avoid the cluttter by
using constructs that promote constants to proper proto terminals.

Any advice?

Thanks!

                              -Dave