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 09:58:06

(Please don't top-post. Rearranging...)

On 1/16/2011 9:36 PM, Hossein Haeri wrote:
> On 16/1/11, Eric Niebler wrote:
>> On 1/15/2011 6:31 PM, Hossein Haeri wrote:
>>> 
>>> 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.

<snip>

> 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?

As I said, "It's a short hop from here to where you want to be, using
is_callable_with_args."

I've gotten you 90% of the way there. The rest is an exercise for the
reader.

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