Re: [boost] Re: MPL usage for code generation

From: David Abrahams (dave_at_[hidden])
Date: 2003-01-23 23:31:16

"Joel de Guzman" <djowel_at_[hidden]> writes:

> ----- Original Message ----- 
> From: "Aleksey Gurtovoy" <agurtovoy_at_[hidden]>
>
>> My current understanding (which, admittedly, is not backed up by a
>> real-world experience) is that if you care about higher-orderness of your
>> generic algorithms, a preferred implementation construct for those
>> algorithms is not a function template, but a static _function object_ (a
>> technique used in FC++):
>> 
>>     struct my_function_
>>     {
>>         template< typename U >
>>         void operator()(std::string const& text, U)
>>         {
>>             // ...
>>         }
>> 
>>     } my_function; // here!
>> 
>> 
>> For ordinary uses, the above will act just like a plain function template
>> (minus ADL/explicit template arguments specification):
>> 
>>     my_function("text", int());
>> 
>> and it will also allow one to do something like this:
>> 
>>     std::string text("text");
>>     mpl::for_each< my_types >(boost::bind<void>(my_function, text, _1));
>
> This technique is adopted by Phoenix. With this, you can even do 
> (as suggested by Joel Young):
>
>         -  \         \
> double  -  /\  f .   /\   x.  f(f x)
>         - /  \      /  \
>
> struct square_ {
>
>     template <typename X>
>     struct result { typedef X type; };
>
>     template <typename X>
>     X operator()(X x)
>     {
>         return x * x;
>     }
> };

This is similar to my "universal identity" suggestion from many months
back.  However, "result" should be "apply" to make it an MPL
metafunction class <wink>.

Hum, that result template looks like identity, and not square. Am I
missing something?  I'd have thought:

    template <typename X>
    struct result :  mpl::times<X,X> {};

> function<square_> square;
>
> template <typename F>
> struct ffx {
>
>     template <typename X>
>     struct result { typedef X type; };

Mighty confused.  Not

    template <typename X>
    struct result :  mpl::apply<F, typename mpl::apply<F,X>::type> {};

??

If not, what's the point of "result"?

> PS> Jaakko and I are working on the LL/Phoenix merger.

Yay!  What about my tuples question?

-- 
                       David Abrahams
   dave_at_[hidden] * http://www.boost-consulting.com
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