Didn't know there's a flag.
Why isn't it enabled by default?
Seems like a no-brainer optimization to do when the conditions allow it.
Also, it works in all explicit/implicit return positions, right? Not just at the end of the function?
What about mutual recursion with both functions calling each other in tail position?
There are implicit trade-off, such as losing the stack trace which could make things harder to debug.
I think you have to keep in mind though, software is about tradeoffs - just because the condition allow it does not mean the tradeoff is worth it.
One of the most popular use case is recursion, but we don't really do recursions in Ruby, so is it really worth the tradeoff then?
It's also Python's reasoning (Guido). You can read more about his blog posts here explaining his rationale, it's a pretty good read.
You can also read about the progress on Ruby's official issue tracker - bugs.ruby-lang.org/issues/6602
Yes it doesn't have to literally be on the last line, but just the last executed statement.
Not sure what you mean about mutual calling, I don't think that's really possible (I could be wrong) - Yes, you can mutual call each other at tail call position, but that does not instantly make it possible for tail call optimization; TCO is when a compiler recognizes it's calling itself (as stated in the gifs). TCO also eliminates the stackframe (can read in the issue tracker). I don't think compilers can recognize a mutual calling tail call and optimize that
The tradeoffs for Ruby specifically sound like something that is more relevant in development than production, almost like something that should be a flag... Oh. That's what they did. Okay.
(I'd still suggest it be the default, until someone needs to pass --debug, but I get the idea)
ES6 spec mandated TCO for mutually recursive functions as well that both call each other in tail call position.
Only Apple browsers have it implemented at the moment, Chrome implemented it but later removed it.
One notable part of the spec was that
if (n<=0) return
would not get optimized, because there is an implicit return undefined after the last line.
I understand how that limitation came about, but I don't really see why they couldn't add a single "void" to the stack and then merrily recurse as if it's this:
if (n<=0) return
since the return value of the inner a is never used.
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