This is funny because I was heavily skeptical yet completely reversed my opinion within 10 minutes. I correctly predicted the results, but I program R professionally. Then I asked my partner who's a non-programmer to read the snippet and her prediction was that the program would stop. That led me to reflect back on your statement. I think the key word is "sharp". I correctly predicted the result, but that's only because I trust lazy-eval heavily. Even then, it's hard to really cement trust in lazy-eval (for me) as it's difficult to mentally map. That's not exactly sharp after all!

>My experience asking people about that snippet is that very few people can correctly predict the results of that function without actually executing the code.

Which is still not any serious barrier in using R succesfully.

Lazy evaluation in R isn't that difficult to grasp once you accept the fact and don't superimpose an evaluation model dominant in most other languages. It turns out, this kind of lazy evaluation can actually be quite useful.

My first answer was correct, although i'll admit to having run the snippet afterwards as I figured there was some trick in there that I couldn't see.