[Rd] mean(x) for ALTREP

[Serguei Sokol]

Hi,

By looking at a doc about ALTREP 
https://svn.r-project.org/R/branches/ALTREP/ALTREP.html (by the way 
congratulations for that and for R-3.5.0 in general), I was a little bit 
surprised by the following example:

 > x <- 1:1e10
 > system.time(print(mean(x)))
[1] 5e+09
    user  system elapsed
  38.520   0.008  38.531

Taking 38.520 s to calculate a mean value of an arithmetic sequence 
seemed a lot to me. It probably means that calculations are made by 
running into a for loop while in the case of arithmetic sequence a mean 
value can simply be calculated as (b+e)/2 where b and e are the begin 
and end value respectively. Is it planned to take benefit of ALTREP for 
functions like mean(), sum(), min(), max() and some others to avoid 
running a for loop wherever possible? It seems so natural to me but 
after all some implementation details preventing this can escape to me.

Best,
Serguei.