for loop operator

[paciorek]

@perrydv do we want to support non-sequential for loop constructions such as for(k in c(2,3,5)) or for(k in values) or for(k in c('foo','bar'))?

I think I roughly see how to set this up in terms of creating a local variable, finding its length, and then assigning into the index variable as the first (new) line of the body of the loop.

[paciorek]

I'll note we do have a basic for loop operator based on : ranges set up. @perrydv had thought that wasn't created.

[perrydv]

Thanks @paciorek. I think the range-based for loops in C++ can come closer to those more R-like constructions, but there are some not-so-corner cases that could cause trouble. I think it is basically another case of bridging the totally different copy vs reference semantics (e.g. potentially different behavior if values is modified within the loop). One idea would be to introduce a new nFor syntax that can handle some specifications. There is also a question of whether to live with nimble compatibility for now or go for more features at the outset. Worth a discussion.

[paciorek]

Yeah, I am all for staying focused. I don't think we should do it if it would pull your time/attention. OTOH if I had time and it's something I could do, particularly if it helped me learn some internals, it might be worth it. Not sure.