So Just What is a Policy Anyway?

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#### So Just What is a Policy Anyway?

A policy is a compile-time mechanism for customising the behaviour of a special function, or a statistical distribution. With Policies you can control:

- What action to take when an error occurs.
- What happens when you call a function that is mathematically undefined (for example if you ask for the mean of a Cauchy distribution).
- What happens when you ask for a quantile of a discrete distribution.
- Whether the library is allowed to internally promote `float` to `double` and `double` to `long double` in order to improve precision.
- What precision to use when calculating the result.

Some of these policies could arguably be runtime variables, but then we couldn't use compile-time dispatch internally to select the best evaluation method for the given policies.

For this reason a Policy is a type: in fact it's an instance of the class template `boost::math::policies::policy<>`. This class is just a compile-time-container of user-selected policies (sometimes called a type-list):

    using namespace boost::math::policies;
    //
    // Define a policy that sets ::errno on overflow, and does
    // not promote double to long double internally:
    //
    typedef policy<domain_error<errno_on_error>, promote_double<false> > mypolicy;