Re: [boost] Egg review result

From: shunsuke (pstade.mb_at_[hidden])
Date: 2008-05-20 03:10:55

Daniel Walker wrote:
>> Egg experimentally contained a multi-signature function adaptor.
>> Here is an example: http://tinyurl.com/68luzd
>> It doesn't perform type-erasure, so I'm not sure there is an overlap.
> 
> I looked at it briefly and noticed the term overload_set showed up in
> your implementation as well. I think this must be a recurring pattern
> - some sort of on-fly set of closely related functions differentiated
> by their argument types and arity. Also, I noticed we both came up
> with a similar way of representing the signatures of polymorphic
> function objects using result_of-style signatures with
> mpl::placeholders! That has got to be a good sign!

egg::overload makes use of placeholders for argument pattern matching.
It doesn't mean any signature.

> The difference is that your Egg-based version cannot handle arbitrary
> callable objects (builtins as well as function objects).

Egg works with result_of protocol, which supports function-pointers.

> Also, I
> didn't notice that you have any mechanism for switching out the
> overloads - an insertion or assignment operation, something more
> set-like than function-like. I'd like to see us move towards something
> along the lines of the following, and I think we're getting there.
> 
> template<class T>
> T f(T t) { return t; }
> 
> struct g {
>     template<class> struct result;
>     template<class T> struct result<g(T)> {
>         typedef T type;
>     };
>     T operator(T t) { return t; }
> };
> 
> overload_set<char(char), int(int), g(_1)> h;
> h.insert(&f<char>);
> 
> // type-erasure bound to int(int)
> h.insert(function<int(int)>(g()));
> 
> // polymorphic g bound to g(_1)
> h.insert(g());

BTW, what if `g(_1)` means templated function returning `g` type?

> // rebind int(int) to f
> h.insert(&f<int>);
> 
> // dispatch to the various overloads
> h('a');
> h(0);
> h("foo");
> 
> I have a polymorphic_function (well, I have the boilerplate for a
> unary version in an attachment on another thread; I'm
> Boost.Preprocessor metaprogramming the nary version now.), which takes
> care of promoting the builtins and interpreting both
> boost::function-style call signatures and result_of-style
> "polymorphic" signatures with placeholders. overload_set can be built
> on top of it. I'm not sure that your version can help with this
> because it seems to be tightly bound to the whole Egg framework.
> However, it would be nice if overload_set could work with fusion,
> lambda, etc. out-of-the-box, and I believe Egg has ways of doing that.
> Could that functionality be made accessible without having the whole
> framework piggybacking along? Other thoughts?

I'm reading that thread.
The motivation and role seems different.
For example,

struct IfInt { ... }; // a function object type
struct Otherwise { ... }; // a function object type

/* It is cumbersome to write down something like this:
struct F_
{
     void operator()(int x) { IfInt()(x); };
     void operator()(any x) { Otherwise()(x); };
};
*/
// Hence, Egg provides a helper to build overloaded function in one shot.
static_<result_of_overload<IfInt(int), Otherwise(_)>::type>::type
    const f = {{}}; // BTW, f is empty.

// Now, if type-erasure is needed...,
multi_signature_function<void(int), void(double)> msf = f;

Egg doesn't touch a job of multi_signature_function.
In fact, I've never used anything like multi_signature_function,
so, I probably can't advise you.
As suggested, I feel container-like behavior is not a job of multi_signature_function, though.

Regards,

-- 
Shunsuke Sogame