Re: [boost] Phoenix review

Subject: Re: [boost] Phoenix review
From: Joel de Guzman (joel_at_[hidden])
Date: 2008-09-27 00:23:36

Giovanni Piero Deretta wrote:

[snip about the switch_]

> Hum, what about relaxing a little the 'C++ in C++ tag' and making
> switch_ more powerfull than the builtin switch?
> 
> Instead of:
> 
>     switch_(arg1)
>     [
>         case_<1>(cout << val("one") << '\n'),
>         case_<2>(cout << val("two") << '\n'),
>         default_(cout << val("other value") << '\n')
>     ]
> 
> do this:
> 
>     switch_(arg1)
>     [
>         case(1)[cout << val("one") << '\n'],
>         case(2)[cout << val("two") << '\n'],
>         default_[cout << val("other value"] << '\n')
>     ]
> 
> this allows not only runtime selection of switch case values, but it
> will also work with everything that is equally comparable:
> 
>     switch_(arg1)
>     [
>         case("one")[cout << val(1) << '\n'],
>         case("two")[cout << val(2) << '\n'],
>         default_[cout << val("other value"] << '\n')
>     ]
> 
> which IMHO is a killer feature.

Cool!

> The fact that the case parameter is not a template parameter might
> limit compiler optimizations unless the the compiler is good at
> constant propagation, but there is a workaround:
> 
>     switch_(arg1)
>     [
>         case(mpl::int_<1>())[cout << val("one") << '\n'],
>         case(mpl::int_<2>())[cout << val("two") << '\n'],
>         default_[cout << val("other value"] << '\n')
>     ]
> 
> Let's leave the ugly syntax to those that need it :)

It seems your proposal is compatible with this syntax anyway:

     case<1>()[cout << val("one") << '\n'],

So you can choose one or the other.

> As another extension, when visiting heterogeneous sequences (think
> fusion::for_each), a switch by type would be great:
> 
>     switch_(arg1)
>     [
>         case(type<std::string>())[cout << val("std::string") << '\n'],
>         case(type<int>())            [cout << val("int") << '\n'],
>         default_[cout << val("other type"] << '\n')
>     ]
> 
> (in this case switch_ would need to know about type<> because it must
> not compile the not-taken branches). Even better:
> 
>     switch_(arg1)
>     [
>         case(type<std::string>())[cout << val("std::string") << '\n'],
>         case(type<std::vector<_> >())  [cout << val("a vector of
> something") << '\n'],
>         default_[cout << val("other type"] << '\n')
>     ]

I love it! I think it's wonderful. Would you be interested in
implementing this? :-P

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net