Re: [Boost-users] [fusion] proposal BOOST_FUSION_ADAPT_FUNCTION, BOOST_FUSION_DEFINE_FUNCTION, etc

Subject: Re: [Boost-users] [fusion] proposal BOOST_FUSION_ADAPT_FUNCTION, BOOST_FUSION_DEFINE_FUNCTION, etc
From: OvermindDL1 (overminddl1_at_[hidden])
Date: 2010-10-15 01:49:49

On Thu, Oct 14, 2010 at 11:45 PM, OvermindDL1 <overminddl1_at_[hidden]> wrote:
> On Thu, Oct 14, 2010 at 11:41 PM, alfC <alfredo.correa_at_[hidden]> wrote:
>>
>>
>> On Oct 14, 10:30Â pm, OvermindDL1 <overmind..._at_[hidden]> wrote:
>>> On Thu, Oct 14, 2010 at 11:09 PM, alfC <alfredo.cor..._at_[hidden]> wrote:
>>> > Hi,
>>>
>>> > Â I am using Boost.Fusion lately. Sometimes one has a function with
>>> > several parameters,
>>>
>>> > int f(double d , int i , std::string s){
>>> > Â ...bla bla ...
>>> > }
>>>
>>> > the logic of the program says that sometimes that same funcion will be
>>> > called by generic code and in such case it is better if "f" where
>>> > defined as taking a fusion::vector as argument.
>>>
>>> > int f(fusion::vector<double, int, std::string>) // another function,
>>> > could be an overload even
>>>
>>> > since there is only one why to define this adaptor function, e.g.
>>> > int f(fusion::vector<double, int, std::string> x){
>>> > Â return f(at_c<0>(x), at_c<1>(x), at_c<2>(x));\
>>> > }
>>>
>>> > and it is pretty mechanical. I was wondering if it would be a good
>>> > idea to have, with consistency with the  BOOST_FUSION_ADAPT_STRUCT,
>>> > something called BOOST_FUSION_ADAPT_FUNCTION
>>>
>>> > BOOST_FUSION_ADAPT_FUNCTION(
>>> > Â  Â int, f,
>>> > Â  Â double,
>>> > Â  Â int,
>>> > Â  Â std::string
>>> > )
>>> > (or something like that)
>>> > that defines the second / fusion-compatible version of f.
>>>
>>> > It could be even a
>>>
>>> > BOOST_FUSION_DEFINE_FUNCTION(
>>> > Â  Â int, f,
>>> > Â  Â (double, d),
>>> > Â  Â (int, i),
>>> > Â  Â (std::string, s),
>>> > Â  Â ( ... code using variables d, i, s or at_key<d_>, at_key<i_>,
>>> > at_key<s_> ... Â )
>>> > )
>>>
>>> > that defines simultaneously the raw-C function, f(double, int, string)
>>> > *and* the fusion friendly version f(vector<...>).
>>> > I could probably program these macro for specific cases but not a
>>> > general one since I don't know enough macro syntax to make it work.
>>>
>>> > Do you think it is a good idea or it is already doable with existing
>>> > Fusion features?
>>>
>>> It should already be doable, given example:
>>> Â  // include fusion here
>>>
>>> Â  int f(double d , int i , std::string s){
>>> Â  Â  ...bla bla ...
>>> Â  }
>>>
>>> Â  int main(void) {
>>> Â  Â  boost::fusion::vector<double,int,std::string myVec(3.14, 42,
>>> "Hello World!");
>>> Â  Â  return boost::fusion::invoke(&f, myVec);
>>> Â  }
>>
>> cool.
>> one question: Â does it work for member functions as well?
>
> It works for any thing that fulfills the Boost.Fusion callable concept
> as outlined here:
> Â http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/functional/concepts/callable.html
>
> So yes, it works for member functions (where the class instance it is
> called from is the first entry in the fusion vector/list/whatever).

Do note, you can also 'fuse' your functions so they are directly
callable by a vector too, but they have to model the deferred callable
concept then:

  http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/functional/concepts/def_callable.html

  http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/functional/adapters/fused.html

So plain functions would not work, but functors would.