[boost] [math distributions] where to check for validity of distribution variables?

Subject: [boost] [math distributions] where to check for validity of distribution variables?
From: Thijs van den Berg (thijs_at_[hidden])
Date: 2008-11-21 09:54:54

I was playing around with implementing the (simple) Laplace distribution 
to get some feeling with 
"math/distributions" code and concepts.

A lot of non-member functions for statistical distributions (like pdf, 
cdf) make a local copy of the distribution internal
variables, and validate those local variables.  An alternative would be 
to move the checking to member function of
the distribution.

instead of

// CREATE LOCAL COPY OF DISTRIBUTION VARIABLES
   RealType sd = dist.standard_deviation();
   RealType mean = dist.mean();
...
// VALIDATE THOSE LOCAL VARIABLES
   RealType result;
   if(false == detail::check_scale(function, sd, &result, Policy()))
   {
      return result;
   }
   if(false == detail::check_location(function, mean, &result, Policy()))
   {
      return result;
   }

do it like this

// VALIDATE DISTRIBUTION
   RealType result;
   if(false == dist.check())
   {
      return result;
   }
// CREATE LOCAL COPY OF VALID DISTRIBUTION VARIABLES
   RealType sd = dist.standard_deviation();
   RealType mean = dist.mean();

I see two benefits:
* the code is more compact this way and repeated code in most of the non 
member
functions is centralized (into the distribution)
* for multivariate distributions, the checking can be expensive (a 
covariance matrix
of a multivariate normal needs to be semi positive definite). We could 
do the checking
only once in the constructor & cache the result.

What do you think? We might turn "having valid parameters"  into a 
property of *all*
distribution. As an alternative, we might add a non member function
bool valid<distributionType...
but that wouldn't allow for caching validation in e.g. a constructor

In general (but in the scope of the constructs  used in 
math/distributions & its non member
functions): what are the arguments for placing code in either member or 
non-member functions ?