On 11/18/2010 6:09 AM, Thomas Heller wrote: > Ok ... I implemented let! You've been busy. :-)
> Here goes the renumbering example: > http://codepad.org/K0TZamPb > > The change is in line 296 rendering RenumberFun to: > struct RenumberFun > : proto::fold< > _ > , make_pair(fusion::vector<>(), proto::_state) > , let< > _a(Renumber(_, second(proto::_state))) > , make_pair( > push_back( > first(proto::_state) > , first(_a) > ) > , type_of<second(_a) > > ) > > > > > {}; > > The implementation of let actually was quite easy ... here is how it works: > > let<Locals, Transform> is a transform taking definitions of local variables > and > the transform these locals will get applied to. > A local definition is in the form of: LocalName(LocalTransform) > If the specified transform has LocalName embedded, it will get replaced by > LocalTransform. Unfortunately, this doesn't actually solve the reevaluation problem ... it just hides it. If "_a" gets replaced eagerly everywhere with "Renumber(_, second(proto::_state))", then that transform will actually get run every time. What I want is a way to evaluate the transform, store the result in a temporary, and have "_a" refer to that temporary. It's REALLY hard. The let context needs to be bundled with the Expr, State, or Data parameters somehow, but in a way that's transparent. I don't actually know if it's possible. -- Eric Niebler BoostPro Computing http://www.boostpro.com _______________________________________________ proto mailing list proto@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/proto
