[boost] [mpl] Integral Constant is over specified

From: Andy Little (andy_at_[hidden])
Date: 2006-07-25 03:39:09

Hi,

 IMO the Integral Constant Concept is over specified in the mpl docs.
 What is the rationale behind the next<..> and prior <..> requirements?

These seem to have little to do with an Integral Constant. Maybe they should
rather be another Concept , such as Iterable?

Where N is a model of Iterable:

next<N>::type
prior<N>::type

Currently for example, mpl::bool_ is stated to be  a model of Integral Constant,
but it fails to meet the curently stated requirments, nor can it ever AFAICS.

Further the requirements currently include a member ::value_type.
According to the TMP book, this is a classic  example of a traits blob. (section 
2.2).
Surely access should be specified using value_type<C>::type?

IOW maybe a ValueType Concept.

Finally the runtime evaluation requirement could be removed and a refinement of 
ValueType
such as  RunTime Evaluable be stated in terms such as:

Where T is a type modelling Runtime Evaluable

value_type<T>::type  = t();

For a type N The current Integral Constant spec would then, in those places
where these requirements are required:

N is a model of Iterable, Integral Constant and Runtime Evaluable.

This would allow me to meet the new Integral Constant requirements without 
unneccesary baggage.

Further a bool_ could be correctly specified in terms of the above Concepts

regards
Andy Little