Re: [Boost-users] [lambda] Help figuring out variable substitution, lazy functions, etc.? Should I be looking at Phoenix

Subject: Re: [Boost-users] [lambda] Help figuring out variable substitution, lazy functions, etc.? Should I be looking at Phoenix
From: Jesse Perla (jesseperla_at_[hidden])
Date: 2009-08-06 08:35:21

On Aug 3, 11:35 am, Steven Watanabe <watanab..._at_[hidden]> wrote:
> AMDG
> Phoenixallows lazy functions like this
>
> struct first_moment_type {
>     typedef double result_type;
>     double operator()(double arg) {
>         return x;
>     }
>
> };
>
> phoenix::function<first_moment_type> first_moment;

Thanks as always Steven,
What is the general thinking on lambda vs. phoenix 2 vs. phoenix 3?
Do we expect phoenix 3 to be the main lambda library in boost
(eventually)?  Should I start getting comfortable with Phoenix 2 to
prepare for it or might Phoenix 3 be radically different?

Also, I can't figure out in phoenix how I would have lazy operator()
on already constructed functors.  Is this possible?

The main use case of this is to use an expectation operator over an
interpolator:
--------------------------------------------
template<typename F, typename Distribution>
double expectation(F f, Distribution dist){
  //Integrates f using dist over its entire support.
  //... monte carlo, quadrature, etc.
}

class linear_interpolator{
  //construction, etc.
  double operator()(double x){....}
};
//How to make operator() lazy here?  I could modify linear
interpolator itself if that helps.

struct g_type {
    typedef double result_type;
    double operator()(double arg) {
        return x;
    }

};
phoenix::function<first_moment_type> g;

 //User code
linear_interpolator f(....);
boost::math::normal_distribution dist(0,1);

using namespace boost::phoenix;
actor<argument<0> > const _x= argument<0>(); //dummy integration
variable
cout << expectation( f( _x), dist);

//And the main reason I want everything lazy is to allow this which
maps
//the mathematical models I deal with.

cout << expectation( f( g(_x) ), dist);

Thanks,
-Jesse