Re: [boost] Template metaprogramming libraries

Subject: Re: [boost] Template metaprogramming libraries
From: David Sankel (camior_at_[hidden])
Date: 2011-09-16 13:09:51

On Fri, Sep 16, 2011 at 6:30 AM, Thomas Heller
<thom.heller_at_[hidden]>wrote:

> On Friday, September 16, 2011 06:17:00 AM David Sankel wrote:
> > On Fri, Sep 16, 2011 at 4:51 AM, Thomas Heller
> >
> > <thom.heller_at_[hidden]>wrote:
> > > On Thursday, September 15, 2011 12:31:50 PM David Sankel wrote:
> > <snip>
> >
> > > <rant>
> >
> > <snip>
> >
> > > It should be noted though that phoenix users can get around
> > >
> > > > this somewhat by using _identifier syntax.
> > > > </rant>
> > >
> > > #include <boost/phoenix.hpp>
> >
> > <snip>
> >
> >    auto flip = lambda(_a = _1)[bind(_a, _2, _1)];
> >
> >
> > <snip>
> >
> > > Thanks for your unqualified rant, David.
> >
> > huh?
>
> Is this not what you were saying wasn't possible with phoenix?
> Having a De Brujin style syntax is perfectly possible in phoenix. It just
> hasn't been done yet cause, as you saw in my example, can be expressed by
> using phoenix local variables.

Oh, my bad. When I said "_identifier syntax", I was referring to phoenix
local variables. I should have double-checked my phoenix vocab before I
wrote.

Phoenix locals definitely help fill the coverage problem I referred to. I
don't know for sure, but I'd expect phoenix to cover all functions that can
be defined in lambda calculus.

The reason I said that phoenix "somewhat" gets around the bind heritage
problems, which was obscure I admit, was because phoenix's use of
traditional bind syntax still leaves a semantic hole.

If someone writes the following:

template< typename F >
auto doSomething( F f ) -> decltype(...)
{
  return bind( f, 15, _1)
}

And then I call doSomething elsewhere like this,

doSomething( bind( g, 12, _1, _2 ) )

, then the result is something most likely unexpected for the user who knows
what doSomething means (it returns a version of f that has 15 filled in as
its first argument (λx. λy. x(15,y))), but hasn't read its implementation.

Don't get me wrong, I think phoenix is an impressive feat. I'm just
advocating that we stop perpetuating the troubles caused by basing new
libraries on the traditional bind syntax.

David
-- 
David Sankel
Sankel Software
www.sankelsoftware.com
585 617 4748 (Office)