Re: [boost] [local] Review

Subject: Re: [boost] [local] Review
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2011-11-21 08:17:18

On Mon, Nov 21, 2011 at 8:07 AM, Joel de Guzman
<joel_at_[hidden]> wrote:
> On 11/21/2011 7:41 PM, Lorenzo Caminiti wrote:
>> On Sun, Nov 20, 2011 at 11:16 PM, Thomas Heller
>> <thom.heller_at_[hidden]> wrote:
>>> On 11/20/2011 09:27 PM, Gregory Crosswhite wrote:
>>>>
>>>> On Nov 21, 2011, at 11:36 AM, Thomas Heller wrote:
>>>>
>>>>> I vote to not include Boost.Local into Boost for the following reason.
>>>>> I obviously can not appreciate the value of local functions in C++.
>>>>
>>>> I similarly have trouble seeing the value of the higher-order functional
>>>> programming constructs provided by Phoenix.  Personally when I want to
>>>> program in a functional programming style I write my code in a language
>>>> Haskell rather than shoehorning functional programming into C++ constructs
>>>> that were never designed for such a purpose.  When I translate my Haskell
>>>> code into C++ (as I have done for a numerical code I once wrote), I would
>>>> never think of using Phoenix, but instead I would change the idioms that I
>>>> was using to match C++'s stateful, imperative nature.
>>>
>>> You should definitely take a look at spirit. Try to use Local in semantic
>>> actions.
>>
>> OK but I'm not at all a Spirit or Phoenix expert so I'll really need
>> your help and guidance. Please note that I'm no Phoenix expert and I
>> know almost nothing about Spirit so this is not aimed to show that
>> Boost.Local could be useful in this context (I have literally no way
>> to predict or assess such usefulness upfront given my sever lack of
>> expertise with Spirit). I'm honestly intellectually interested in
>> discovering the outcome of this exercise together :)
>>
>> Let me start by using Boost.Local to program the semantic action's
>> example from the Spirit docs:
>> http://www.boost.org/doc/libs/1_48_0/libs/spirit/example/qi/actions.cpp
>>
>> #include <boost/local/function.hpp>
>> #include <boost/config/warning_disable.hpp>
>> #include <boost/spirit/include/qi.hpp>
>> #include <iostream>
>>
>> namespace client {
>>
>> namespace qi = boost::spirit::qi;
>>
>> struct print_action {
>>     void operator()(int const& i, qi::unused_type, qi::unused_type) const {
>>         std::cout << i << std::endl;
>>     }
>> };
>>
>> } // namespace
>>
>> int main() {
>>     using boost::spirit::qi::int_;
>>     using boost::spirit::qi::parse;
>>     using boost::spirit::qi::unused_type;
>>     using client::print_action;
>>
>>     { // example using simple function object
>>
>>         char const *first = "{43}", *last = first + std::strlen(first);
>>         parse(first, last, '{' >> int_[print_action()] >> '}');
>>     }
>>
>>     { // example using Boost.Local
>>         void BOOST_LOCAL_FUNCTION_PARAMS(int const& i,
>>                 unused_type, unused_type) {
>>             std::cout << i << std::endl;
>>         } BOOST_LOCAL_FUNCTION_NAME(print_local)
>>
>>         char const *first = "{40}", *last = first + std::strlen(first);
>>         parse(first, last, '{' >> int_[print_local] >> '}');
>>     }
>>
>>     return 0;
>> }
>>
>> This compiles and runs (I can say that much :) ) but a part from that
>> I can't quite comment given my Spirit ignorance. I honestly ask you:
>> What do you think?
>>
>> Notes:
>> 1) Is this too easy of an example? If so, where can I find a more
>> complex one? Maybe in other points of the docs?
>
> Yes, go further down the docs where Phoenix is used.
>
>> 2) I removed the Bind, Lambda, and global function versions
>> thinking/assuming that the comparison with global functor was the most
>> interesting one...
>
> Really? Read what you quoted above. It's about Phoenix, function composition
> and higher order functions.
>
>> was that a mistake?
>
> Definitely, yes. You can't really be seriously dabbling locals into
> FP land now, are you? :-)

Well, I am seriously asking for your guidance entering the
Spirit/Phoenix domain.

Would this be a good example or is there a better one?
http://www.boost.org/doc/libs/1_48_0/libs/spirit/example/qi/num_list4.cpp
There are quire a few examples in the docs even after num_list4 (which
is good) so I can use your help in selecting the best one for this
exercise.

Thanks for the input.
--Lorenzo