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Subject: Re: [boost] [math distributions] where to check for validity of distribution variables?
From: Thijs van den Berg (thijs_at_[hidden])
Date: 2008-11-23 15:10:38

Paul A. Bristow wrote:
>> -----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.
Hi Paul,
thanks for the info!
I'll have to delve into those concepts a bit more I see.
Regarding the checking in non member functions for the validity of the
distribution: would it be possible for the distributin contructor to
fail runtime (before the distribution parameters can be validated)?
Would it be safe for me to assume that
* if a distribution validate its parameters in the constructor
* if the constructor doesn't throw an error
* there is no need to check the distribution parameters anymore after
construction, e.g. in a non-member function.
if so, then I would send in new distributions with only checks in the

another option is to check parameters (and throw errors) in the
distribution parameter access member funtions like "RealType location()
const {return m_location;}". A possible drawback in that is that
sometimes a *combination* of parameters is valid or not..

>> 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
I think the same about that. runtime is good enough, and even
unavoidable if you would allow distribution parameters to be set
runtime. Btw ,why isn't that implemented (allowing distribution
paramters to be set riu-time)? Lack of implementation time (postponed to
future versions), of is it a design choice?

> ---
> Paul A. Bristow
> Prizet Farmhouse
> Kendal, UK LA8 8AB
> +44 1539 561830, mobile +44 7714330204
> pbristow_at_[hidden]
> _______________________________________________
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M.A. (Thijs) van den Berg
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