Re: [boost] Giving names to formal parameters of lambda expressions: a named placeholder trick.

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-04-02 05:30:51

On Wed, Apr 2, 2008 at 1:59 AM, shunsuke <pstade.mb_at_[hidden]> wrote:
> Giovanni Piero Deretta wrote:
>  >>  BTW, Boost.Egg has the potential to implement your compose.
>  >>
>  >>      typedef result_of_lazy<fold>::type fold_;
>  >>      typedef result_of_lazy<reverse>::type reverse_;
>  >>      typedef return_of<fold_(reverse_(T_bll_1), T_bll_2, T_bll_3)>::type reverse_fold;
>  >>
>  >
>  > Oh! So Egg has the functionality to make function objects directly
>  > usable in lambda expressions (without bind)???
>  > i.e.
>  >
>  > for_each(range_of_ranges, regular(protect(std::cout << accumulate(ll::_1, 0))))
>  >
>  > does that actually work? (for appropriate definitions of for_each and
>  > accumulate, of course).
>  > I was going to ask you to provide this functionality in egg (yes, I'm
>  > working on a review :) )
>
>  Yes. egg::lazy/nestN makes bind/protect deprecated.
>  I've noticed that bind/protect is a customization point.
>  e.g.
>
>      X_lazy<T_boost_bind> bb_lazy; // T_boost_bind represents boost::bind.
>      bb_lazy(foo)(::_1, 2);
>
>  In fact, implementing T_boost_bind is so difficult that Egg skips it. :-)
>
>
>  > BTW, i prefer to spell 'regular(protect(...))'  as 'lambda[...]'
>
>  What is `regular`? Is it the same as egg::regular?

yes, exactly. I do not have a 'regular' function, I've just used this
name because this is what egg uses.

>  BTW, everything must be function-call to support result_of.

What do you mean exactly with "everything must be function call?".

>
>
>  > Even if I guarantee that my function objects (in this case fold and
>  > reverse) are stateless? Does using the lambda placeholders complicate
>  > things?
>
>  If fold_ is DefaultConstructible and a default-constructed one is callable,
>  it would be:
>
>  typedef
>      static_< mpl::always<
>          return_of<T_regular(fold_(reverse_(T_bll_1), T_bll_2, T_bll_3))>::type
>      > >
>  reverse_fold;
>
>  I tend to hesitate to use result_of/return_of without function-calls.

Again, what do you mean exactly? And why do you esistate? I'm
evaluating egg design, so these answers would be very valuable.

>  So your `compose` may be a candidate of yet another lambda framework.

As if there weren't already enough :)

-- 
gpd