Binary hybrid evaluation handlers

[romainfrancois]

We now have all the scaffolding to add binary hybrid handlers, e.g. +, -, == at least handling scalar values, so that we can hybridify something like this:

sum( x < 3 )

We know how to handle sum, but since we can't hybrid x<3 the hybrid version of sum is not used.

[hadley]

@romainfrancois do think you can tackle this with the other hybrid evaluator changes? Or should we push off to a future version? I'm aiming to get dplyr 0.3 to CRAN by mid-September.

[romainfrancois]

For this to really be useful, we would have to specialize precisely for each case, i.e. at the point where we see the expression sum( x < 3 ) we would instantiate a class like Sum_LowerThan.

Otherwise, within hybrid eval, we would first substitute x < 3 to some logical vector and then sum that which demands more memory and runtime .

[hadley]

Let's put this off until (at least) 0.4

[hadley]

I'm going to close this for now. I think we should come back to again in the future if we look for potential performance gains.