Re: Spirit-X3 | Boost 1.59 | Compilation never finishes for a recursive grammar

Arun Muralidharan wrote
> Thanks a lot, Larry. I could compile it using the code from your branch. 
> So, the fix would be available now only in boost 1.65 ?
> 
> 
> Thanks.

On 04/22/2017 09:19 AM, Arun Muralidharan wrote:
> 
> Thanks a lot, Larry. I could compile it using the code from your branch. 
> So, the fix would be available now only in boost 1.65 ?

Glad to help, Arun.

However, I'm not in charge of making changes to spirit;
hence, Sehe would be the person to ask.  

I could issue a pull request for get_rhs; however, get_rhs
is somewhat experimental.  It has a patch for another spirit
problem with attribute transforms:

https://github.com/cppljevans/spirit/blob/get_rhs/include/boost/spirit/home/x3/nonterminal/detail/rule.hpp#L383

and I'm guessing that the official maintainers want to keep
patches focused on a single problem.  I'm not even real
certain the attribute transform patch is completely
correct.  That patch may solve one problem but cause another
:(  I need better understanding of how x3 handles attributes
before I'm comfortable with the attribute transform patch.

The problem your code highlighted was that of a particular
rule recursion implementation.  The purpose of the code
linked-to in my OP:

https://github.com/cppljevans/spirit/blob/get_rhs/include/boost/spirit/home/x3/nonterminal/detail/rule.hpp#L215

(i.e. the call to make_rule_context) was to implement rule
recursion by lookup of the rhs of a rule in the context.
Apparently that method works in some cases; however, in
other cases, in particular, the case of your code, it
results in, what I would guess, is infinite template
instantiation.

I could make another branch which only solves this
make_rule_context problem, if someone who is responsible for
making changes to spirit would think that's the way to go.
However, that's gonna break some code.  In particular:

https://github.com/boostorg/spirit/blob/master/test/x3/rule1.cpp

The solution for those breaking cases would be using the
BOOST_SPIRIT_DEFINE method for implementing rule recursion
instead of the make_rule_context method.  This one-method
for rule recursion position was advocated in the .txt file
attachment to:

 
http://boost.2283326.n4.nabble.com/x3-devel-why-2-methods-linking-rule-to-RHS-td4688021.html

However, that post got no interest.

So, there are pro's and con's to the patch, but, hopefully,
your code example may generate more interest in the
one-method rule recursion design.

Sorry I can't give a more definite answer :(

-Larry






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