Brian McNamara <lorgon <at> cc.gatech.edu> writes:
On Wed, Apr 07, 2004 at 08:59:42PM -0400, Arkadiy Vertleyb wrote:
I am wonderring if there is an easy way to determine the type of a lambda functor, kind of lambda-specific typeof facility. I do realize that the primary purpose of lambda is to create temporary functors to pass to STL algorithms. On the other hand, there are contexts where functors need to be stored, such as in std::set, etc. One might want to use Lambda to generate such a functor, but without knowing its type it would be of limited use (of course one can directly specify the type of such functor, but this would be rather tedious).
Has anything been done by anybody in this regard?
I don't know that Lambda has anything like this. FC++ does:
http://www.cc.gatech.edu/~yannis/fc++/boostpaper/fcpp.sectlambda.html#id2708...
I think the possibility of having the type of a lambda expression is extremely interesting. IMHO, this facility can be conceptualized as a "static lambda" library. Let me ellaborate: Consider std::sort. This function takes an extra functor in charge of doing the comparison work, for which Boost.Lambda nicely fits the bill: std::sort(it0,it1,_1<_2); // trivial comparison. std::set, OTOH, also takes a comparison predicate, though this is commonly passed by type rather than as an expression: std::set<int,std::less<int> > Here, we can consider std::less as the static version of _1<_2, with the difference that the <int> has to be explicitly stated. Generally speaking, a given class template T is the static version of a lambda expression expr if T<Arg1,...Argn>()(arg1,...,argn) is the same as expr(arg1,...,argn) for argi of type Argi. So it would be nice if Boost.Lambda is augmented with more or less comfortable syntax to produce static lambda types. I'm thinking of having something as namespace sl=boost::lambda::static; std::set< int, sl::less<sl::_1,sl::_2>::type // equivalent to _1<_2 <int,int> // instantiated for int types
;
std::set< int*, sl::less< sl::deref<sl::_1>, // eq. to *_1<*_2 sl::deref<sl::_2>
::type <int*,int*> // instantiated for int* ;
Maybe this is quite a lot of work, but IMHO the potential of it is great. I don't know wheter MPL has already the possibility of doing something like this (I guess it doesn't), or on the contrary this static lambda library can be regarded as a cooperative add-on to MPL. Comments? Joaquín M López Muñoz Telefónica, Investigación y Desarrollo