It is, indeed, a fundamental goal of ruby that there are multiple ways to write the same thing, and that the programmer should not need to understand nuances of the compiler.
"I need to guess how the compiler works. If I'm right, and I'm smart enough, it's no problem. But if I'm not smart enough, and I'm really not, it causes confusion. The result will be unexpected for an ordinary person. This is an example of how orthogonality is bad." -matz 2003.
In this particular example, the fact that assignments return the value of the right-hand side is well-known and used frequently in Ruby code. The fact that arrays have to be allocated is obvious. The fact that allocations have a runtime cost is obvious. The only thing that isn't obvious is that the return value allocation of assignments whose value isn't used are optimized away. If you know that, you'll think of appending the nil to activate that optimization. Characterizing the lack of that step as a "mistake" only makes sense if the goal for your code is to maximize performance -- which in this case, most unusually for Ruby, it was.
~/scripts > ruby fib.rb
2.3346780000720173
~/scripts > ruby --yjit fib.rb
1.5913339999970049
Running the original fib benchmark (i.e., without the author's technique to eliminate the array allocation) on an M1 Pro, I see:
CRuby 3.3.1:
2.058589000022039
CRuby 3.3.1 w/ YJIT:
1.4314430000958964
TruffleRuby 24.0.1 (Native):
0.20155820800573565
TruffleRuby 24.0.1 (JVM):
0.1336908749944996
Running the new prime_counter benchmark that the crystalruby author mentions in another thread¹, I see:
Crystal 1.12.1 (LLVM 18.1.4) w/ crystalruby 0.2.0 in CRuby 3.3.1:
0.34096299996599555
CRuby 3.3.1:
2.9615250000497326
CRuby 3.3.1 w/ YJIT:
1.640430000028573
TruffleRuby 24.0.1 (Native):
0.2504862080095336
TruffleRuby 24.0.1 (JVM):
0.25282600001082756
¹ -- https://news.ycombinator.com/item?id=40153218
² -- The method_source gem used by crystalruby catches exceptions and matches against the message² for some conditional handling. TruffleRuby 24.0 now uses to Prism as its parser and Prism has an exception message with slightly different wording from CRuby. Consequently, method_source's handling doesn't work with Prism. It's hard to say where the compatibility issue lies, since exception messages aren't stable APIs. We'll get it sorted out.
³ -- https://github.com/banister/method_source/blob/06f21c66380c6...
Ruby code, perhaps more than most code, is written for readability and “beauty”. It’s a part of Ruby culture that I greatly appreciate. But if you care about performance, you will act differently, regardless of language. And the whole point of this code is to show that if you care about performance above all else, there’s of plenty of room to maneuver in interpreted Ruby.