Re: [boost] Template metaprogramming libraries

Subject: Re: [boost] Template metaprogramming libraries
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2011-09-16 04:51:34

On Thursday, September 15, 2011 12:31:50 PM David Sankel wrote:
<snip>
> I'm the one who designed the DeBruijn bind syntax and implementation that
> Dave A and Larry referenced. The DeBruijn bind syntax would work for compile
> time or run time. The sample implementation was for run time.
> 
> DeBruijn bind syntax can represent strictly *more* functions than the
> traditional bind syntax, even with protect. Furthermore, the functions that
> DeBruijn bind can define, that traditional bind cannot, are useful and
> interesting.
> 
> An example is the flip function:
> 
> haskell syntax:
> 
> flip :: ((a,b) -> c) -> ((b,a) -> c)
> flip f (x,y) = f (y,x)
> 
> intuition:
> 
> flip is a function that takes in a two-argument function and returns a
> two-argument function that is essentially the same, but the arguments are
> reversed.

<snip>

> <rant>
> Unfortunately traditional bind syntax and it's inherent design flaws has
> been replicated in several other libraries like mpl, lambda, phoenix, and
> C++-0x (arg!). It should be noted though that phoenix users can get around
> this somewhat by using _identifier syntax.
> </rant>

#include <boost/phoenix.hpp>

#include <iostream>

namespace phx = boost::phoenix;
int main()
{
    using phx::placeholders::_1;
    using phx::placeholders::_2;
    using phx::local_names::_a;
    using phx::local_names::_b;

    using phx::val;
    using phx::lambda;

    auto f = std::cout << _1 << _2 << "\n";

    auto flip = lambda(_a = _1)[bind(_a, _2, _1)];

    auto flipped_f = flip(f);

    flipped_f("David.", "Thanks for your unqualified rant, ");
}

Output:

Thanks for your unqualified rant, David.