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

Subject: Re: [boost] [math distributions] where to check for validity of distribution variables?
From: Paul A. Bristow (pbristow_at_[hidden])
Date: 2008-11-23 05:33:59

> -----Original Message-----
> From: boost-bounces_at_[hidden] [mailto:boost-bounces_at_[hidden]]
On
> Behalf Of Thijs van den Berg
> Sent: 22 November 2008 14:48
> To: boost_at_[hidden]
> Subject: Re: [boost] [math distributions] where to check for validity of
distribution
> variables?
> 
> John Maddock wrote:
> > Thijs van den Berg wrote:
> >>>>>> 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
> >>>>
> >>>> Sounds fine to me.
> >>> thats great! What's your opinion on the fact that you can only set
> >>> parameter in the constructor?
> >>> E.g. the normal distribution does a parameter check in the
> >>> constructor, and those parameters can't change after that.
> >
> > That's what the existing distributions do.  In fact we could omit most
> > of the subsequent parameter checking code if we could figure out
> > whether the error handlers will throw or not on error (in fact we
> > *can* get this information at compile time and make the subsequent
> > checks a no-op if we know that the constructor would have thrown on
> > error... we just ran out of time on that refinement).
> >
> I don't understand this, it has to do with my lack of knowledge on this...
If you ensure
> that the parameters get checked in the constructor, why would that check
*not* throw
> an error when needed?

Often you just want to return a NaN, infinity or a 'best guess'.

So John devised the rather complicated - but very useful - policies.

Most important they are needed to provide the C++ Standard library C-style
error behaviour.

enum error_policy_type
{
throw_on_error = 0, // throw an exception.
errno_on_error = 1, // set ::errno & return 0, NaN, infinity or best guess..
ignore_error = 2, // return 0, NaN, infinity or best guess.
user_error = 3 // call a user-defined error handler.

> Compile time might be tricky depending on the complicity of the parameter
validation
> code, but simple range check on the parameters could be done compile time.
What
> mechanism are your thinking about regarding compile checking, e.g. that
scale>0?

The complexity of policy options make it much simpler to do a run-time
check.

You'd save a tiny bit on run-time - but probably pay in compile time?

Paul

---
Paul A. Bristow
Prizet Farmhouse
Kendal, UK   LA8 8AB
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