Re: [boost] [Proto] Review

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-03-28 17:10:58

AMDG

Eric Niebler wrote:
>>>>   - Does ``and_< T0,T1, ... Tn >`` *really* only apply ``T``\ *n*?
>>>>         
>>> Yes. It can't apply all the transforms. The result of applying the first 
>>> transform might have a type that the second transform can't make sense 
>>> of. Transforms don't chain that way.
>>>       
>> They could be chained by and_, could they not?
>>     
>
> No, not in any meaningful way. Consider:
>
>    and_<
>      when<G1, T1>
>     ,when<G2, T2>
>    >
>
> T1 requires that any expression passed to it matches G1. Likewise for T2 
> and G2. Now, imagine that the result of applying T1 is some type that 
> *doesn't* match G2 ... it might not even be an expression! If you tried 
> to pass it to T2, you would be violating T2's preconditions. T2's 
> transform will likely fail.
>   

How about passing it the _state parameter?

In Christ,
Steven Watanabe