Re: [boost] [mpl] multiset

Subject: Re: [boost] [mpl] multiset
From: Eric Niebler (eniebler_at_[hidden])
Date: 2015-03-12 22:48:23

On 3/12/2015 5:05 PM, Louis Dionne wrote:
> Eric Niebler <eniebler <at> boost.org> writes:
>> [...]
>>
>> using factorial_ = lambda_rec<_a,
>>     lazy::if_<lazy::greater<_a, meta::size_t<0>>,
>>               lazy::multiplies<_a, lazy::apply<_self, lazy::dec<_a>>>,
>>               meta::size_t<1>>>;
>> template<std::size_t N>
>> using factorial = apply<factorial_, meta::size_t<N>>;
>>
>> Metafunctions? We don't need no stinkin' metafunctions! 
> 
> FWIW, here's how you can implement a factorial with lazy
> metafunctions, here done with MPL11:
> 
>     template <typename N>
>     struct fact
>         : if_c<N::value == 0,
>             ullong<1>,
>             mult<N, fact<pred<N>>>
>         >
>      { };
> 
> Lambda expressions are a nice hammer, but a factorial metafunction 
> is seemingly not a nail.

Ha! That works in Meta too:

using namespace meta;
template<typename N>
struct factorial : eval<
    if_c<N::value == 0,
         meta::size_t<1>,
         lazy::multiplies<N, factorial<dec<N>>>>>
{};

But now that I think about it, there *is* a difference between MPL-style
lazy metafunctions and meta::lazy that could complicate these uses of
Meta. The metafunctions in meta::lazy don't assume their arguments are
themselves lazy metafunctions, which would make lazy metafunction
composition difficult. It's really just not the Meta way.

-- 
Eric Niebler
Boost.org
http://www.boost.org