Re: [boost] painless currying

Subject: Re: [boost] painless currying
From: Larry Evans (cppljevans_at_[hidden])
Date: 2011-09-03 09:28:02

On 08/23/11 14:38, Eric Niebler wrote:
> After playing around with functional languages, I've come to envy how
> easy they make it to curry functions. Call a 2-argument function with 1
> argument and you get a function that takes 1 argument. Pass another
> argument and it evaluates the function. Simple. In contrast, C++ users
> have to use binders, which are not as nice.
> 
> On a lark, I implemented a wrapper that turns any TR1-style function
> object into a "curryable" function object (attached). Successive
> function call invocations bind arguments until enough arguments are
> bound to invoke the wrapped function. With it you can do the following:
> 
>     curryable<std::plus<int> > p;
>     auto curried = p(1);
>     int i = curried(2);
>     assert(i == 3);
> 
> Is there any interest in such a thing?

With the 2 attached files (compiled with gcc4.6 and with boost trunk
[which svn update showed was revision 74198]), the output is:

/home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_6v/boost-svn/ro/trunk/sandbox/rw/variadic_templates/sandbox/painless_currying/curryable.exe

type_u<0>()
type_u<0>(type_u const&)
type_u<0>(type_u const&)
type_u<1>()
type_u<1>(type_u const&)
type_u<0>(type_u const&)
type_u<0>(type_u const&)
type_u<1>(type_u const&)
type_u<2>()
sout<<type_u<0>
sout<<type_u<1>
sout<<type_u<2>

Compilation finished at Sat Sep  3 08:20:45

which shows several copy CTOR's executed.  These copy CTOR
calls were caused by the code like this:

  curryable (Fun f, Arg0 a0, Arg1 a1, Arg2 a2):
  fun (f),
  arg0 (a0),
  arg1 (a1),
  arg2 (a2)
  {
  }

produced by the BOOST_PP code.  Is there any way to prevent
these copies, maybe by declarations like:

  Arg0 const&
    arg0;

instead of the existing:

  Arg0
    arg0;

?

-regards,
Larry