Re: [Boost-users] [fusion] named parameter technique with fusion::map

On Sat, Nov 6, 2010 at 2:19 PM, Dave Abrahams wrote:

On Sat, Nov 6, 2010 at 5:19 AM, alfC wrote:

...

(Note, named parameter boost library is not the proper way to go here because the of the level of instrospection I need in the functions, and because the use of named parameters is not to omit passing parameters but to recognize 'externally' certain parameters).

I would be *very* surprised if there was some kind of introspection possible that Boost.Parameter didn't allow. And I don't understand what you mean about omitting passing parameters or external recognition. I think you're reinventing the wheel; a Boost.Parameter ArgumentPack is essentially a fusion map.

ok, since I am probably missing the point, I am going to be one order of magnitude more specific in defining what I want to achieve. In the following example I will mix C++ and mathematical language to make the point clear. In "math", one can define a function of parameters and with some abuse of notation one can use the names of parameters to define functionals (e.g. derivatives), so for example H(p,q) = p*p + q; which can be evaluated, e.g. H(1.,2.) == 3. but one can also "invoke" the derivatives of H, dH/dq(1.,2.) == 4. To be consistent with the abuse of notation one can also say H(p=1., q=2.) or even H(q=2.,p=1), etc. It is this degree of notational flexibility what I would like to achieve. Let's switch to C++ double H(double p, double q){ return p*p+q; } if I call H(1.,2.) I get of course the desired result. I would also like to call something like d(1.,2.) which is the numerical derivative of H with respect to q evaluated at p=1., q=2. That is not possible with the naive definition of H because "q" means nothing in the template parameter of "d". So the next step is to inform C++ of the naming of the parameters, so I did: struct p{}; struct q{}; double H(map, pair > const& args){ return at_key<p>(args)*at_key<p>(args) + at_key<q>(args); // same as return H(at_key<p>(args), at_key<q>(args)); } great, now the compiler has some idea of what the parameters are. I can call this function as H(make_map(1.,2.)); H(make_set(1.,2.)); or even as H(1.,2.) now the derivative problem: suppose I already have a magical templated function, called 'derivative' that returns the numerical derivative of a function or function object, when this function has only one argument. template double derivative(double const& x){ ...already defined... } double f(double x){return x*x;} used as derivative<f>(1.); the question is how to define a function "d" that takes a function like H with fusion container arguments, bind it against all arguments except one and calls derivative on that function. For example d(1.,2.); internally, the code should bind all the parameters except q to its corresponding values in the argument. in this case it is H binded with p=1. and then *derivative* of H( __, 2.) is called with argument 1. Christofere pointed example libs/fusion/example/cookbook/do_the_bind.cpp seems to be in the right direction but the code is hard to read, not to mention that it doesn't compile. Thank you, Alfredo

-- Dave Abrahams BoostPro Computing http://www.boostpro.com _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users

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