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

Subject: Re: [Boost-users] [Proto] Difficult Arity Check
From: Hossein Haeri (powerprogman_at_[hidden])
Date: 2011-01-16 09:36:36

Dear Eric,

This is great, thanks. But, it's still not quite what I'm after; what wasn't obvious from my last explanation was that I don't want to restrict these f's and g's to only functions (function pointers). I'm about to also include function objects, or, in fact, whatever callable with that arity.

Any idea now?

TIA,
--Hossein

--- On Sun, 16/1/11, Eric Niebler <eric_at_[hidden]> wrote:

> From: Eric Niebler <eric_at_[hidden]>
> Subject: Re: [Boost-users] [Proto] Difficult Arity Check
> To: boost-users_at_[hidden]
> Date: Sunday, 16 January, 2011, 10:31
> 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
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