Re: [Boost-users] [Proto] Difficult Arity Check

Subject: Re: [Boost-users] [Proto] Difficult Arity Check
From: Eric Niebler (eric_at_[hidden])
Date: 2011-01-16 05:31:22

On 1/15/2011 6:31 PM, Hossein Haeri wrote:
> Dear all,
> 
> I have a syntax like
> 
> ((_1 >> &f1 >> &f2) || (_2 >> &f3) || (_3 >> &f4 >> &f5 >> &f6)) >> g
> 
> for which I already have a small grammar which along with my context
> do their job fine. The above expression is to act like
> 
> g(f2(f1(x)), f3(y), f6(f5(f4(z))))
> 
> once invoked with _1 == x, _2 == y, and _3 == z. I am currently
> trying to add a safety layer on top of that which can emit a
> compile-error -- at the appropriate corner of my code -- that can
> prevent pass of functions with wrong arities. For example, g above
> needs to be a ternary.
> 
> Although my context will notice it if g is not a ternary and produce
> a meaningful error message, that's too late. My poor user needs to be
> informed right at the instantiation time. So, I thought I'm going to
> use transforms to get my grammar manage that.

<snip>

Below is a grammar Stream that only matches these function stream
expressions where the number of arguments to each callable matches the
arity of the callable. It's a short hop from here to where you want to
be, using is_callable_with_args.

struct FunctionPtr
  : proto::and_<
        proto::terminal<_>
      , proto::if_<is_function<remove_pointer<proto::_value> >()>
    >
{};

struct Stream;

// The grammar for the thing that can appear on the
// LHS of a >> operator, and a transform that computes
// the arity of that expression.
struct StreamArgs
  : proto::or_<

        proto::when<
            proto::terminal<_arg<_> >
          , mpl::int_<1>()
        >

      , proto::when<
            proto::logical_or<StreamArgs, StreamArgs>
          , mpl::plus<
                StreamArgs(proto::_left)
              , StreamArgs(proto::_right)
            >()
        >

      , proto::when<
            Stream
          , mpl::int_<1>()
        >
    >
{};

struct Stream
  : proto::and_<
        proto::shift_right<StreamArgs, FunctionPtr>
      , proto::if_<
            mpl::equal_to<
                StreamArgs(proto::_left)
              , function_types::function_arity<
                    proto::_value(proto::_right)
                >()
            >()
        >
    >
{};

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com