RE: [boost] Meta function variants of mpl::apply_if neccesary?

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-09-19 17:01:19

Terje Slettebø wrote:
> Consider the "factorial" metafunction, again:
> 
> template<class V>
> struct factorial
> {
>   typedef typename V::type Value;
> 
>   typedef typename apply_if<equal_to<Value,value<Value,0> >,
>     value<Value,1>,
>     mul<Value,factorial<prior<Value> > >
>   >::type type;
> }
> 
> I had first planned to look at a comparison with using "eva," 
> but I realized that I had a hard time finding out how I could 
> write the above, using "eva," since the call to factorial is used 
> as a parameter to "mul," and unlike apply_if, that doesn't evaluate 
> its arguments, which is what makes your first "eva" example work.

Here's how you could write it with lazy-lambda:

    template< typename N >
    struct factorial
    {
        typedef typename apply_if<
              equal_to< N, int_c<0> >
            , int_c<1>
            , lazy<
                  multiplies< N, factorial< prior<N> > > 
                >
            >::type type;
    };

It even works :).

Aleksey