[boost] [ANN] type_switch with pattern matching in sandbox

From: Eric Friedman (ebf_at_[hidden])
Date: 2003-10-19 19:27:24

This is something I had been working on about two months ago but didn't 
post to the sandbox until recently.

The basic idea is this:

   switch_( [variant] )
     |= case_< [pattern1] >(...)
     |= case_< [pattern2] >(...)
     :::
     |= default_(...)  // optional catch-all
     ;

The switch_ will fail to compile if not every case is handled. In terms 
of handling, the case_ constructors take typical function objects, 
though the switch_ ignores any return values.

An example usage is the following (though I have again left out the 
function objects that need to be passed to the case_ constructors):

   using namespace boost::type_switch;

   variant<
       int
     , pair<int,double>
     , pair<int,int>
     , pair<double,int>
     > var;

   switch_(var)
     |= case_< pair<_1,_1> >(...) // matches pair<int,int>
     |= case_< pair<int,_> >(...) // matches pair<int,*>
     |= case_< pair<_,_> >(...)   // matches pair<*,*>
     |= case_< int >(...)
     ;

The pattern matching is implemented in terms of lambda_match, which I 
have also added to the sandbox (to boost/mpl). Notably, lambda_match 
leverages the MPL lambda workarounds for deficient compilers, extending 
the applicability of type_switch somewhat.

I'd appreciate any feedback on syntax or semantics. My guess is that 
this will appear in the 1.32 release unless feedback is overwhelmingly 
positive.

Thanks,
Eric