Re: [Boost-users] math/statistics policy-interface feedback requested.

From: David Abrahams (dave_at_[hidden])
Date: 2007-06-12 13:44:11

on Tue Jun 12 2007, "John Maddock" <john-AT-johnmaddock.co.uk> wrote:

> define a typedef of the policy class:
>
> using namespace boost::math::policy;
> typedef policy<
>     // Set error handling:
>     domain_error<throw_on_error>,
>     pole_error<throw_on_error>,
>     overflow_error<throw_on_error>,
>     evaluation_error<throw_on_error>,
>     denorm_error<ignor_error>,
>     underflow_error<ignor_error>,
>     // calculate to 8 decimal digits internally
>     digits10<8>,
>     // don't promote double->long double for accuracy
>     promote_double<false>,
>     // Integer quantiles return the "outside edge":
>     // below the real value for lower critical values,
>     // above it for upper critical values, so that the
>     // area inside contains *at least* the requested coverage:
>     discrete_quantile<integer_outside_edge>
>     > fast_quantile_policy;
>
> static fast_quantile_policy fast_quantile;
>
> Then we can just use:
>
> quantile(
>     poisson(100),
>     0.05,
>     fast_quantile);
>
> In our actual code.
>
> Currently this policy interface is vapourware: I have enough of a prototype 
> implemented to know that it's possible to achieve this syntax (this is 
> revision #3 already !), but there's a lot of hairy meta-programming to 
> convert that into something that the library's internals can make use of... 
> so I'd like to know what folks think before I invest too much time messing 
> about with MPL :-)
>
> The main disadvantage I've noticed at present, is that the mangled names of 
> the policy class - and therefore all the special functions etc - are *very* 
> long.  

Why not make it allowable to use derivation:

    struct fast_quantile_policy
      : policy<
        // Set error handling:
        domain_error<throw_on_error>,
        pole_error<throw_on_error>,
        overflow_error<throw_on_error>,
        evaluation_error<throw_on_error>,
        denorm_error<ignor_error>,
        underflow_error<ignor_error>,
        // calculate to 8 decimal digits internally
        digits10<8>,
        // don't promote double->long double for accuracy
        promote_double<false>,
        // Integer quantiles return the "outside edge":
        // below the real value for lower critical values,
        // above it for upper critical values, so that the
        // area inside contains *at least* the requested coverage:
        discrete_quantile<integer_outside_edge>
        >
   {};

?
That will keep your type names under control.

> This has an impact on error messages: in particular we currently use 
> BOOST_CURRENT_FUNCTION to get a nice formatted name of a function that's 
> about to throw, but with function names a couple of pages long I don't think 
> that will be possible with this interface any more :-(
>

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
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