Does Ruby perform Tail Call Optimization?

Question

Functional languages lead to use of recursion to solve a lot of problems, and therefore many of them perform Tail Call Optimization (TCO). TCO causes calls to a function from another function (or itself, in which case this feature is also known as Tail Recursion Elimination, which is a subset of TCO), as the last step of that function, to not need a new stack frame, which decreases overhead and memory usage.

Ruby obviously has "borrowed" a number of concepts from functional languages (lambdas, functions like map and so forth, etc.), which makes me curious: Does Ruby perform tail call optimization?

Answer 1

Sometimes:

http://redmine.ruby-lang.org/issues/show/1256

Answer 2

No, Ruby doesn't perform TCO. However, it also doesn't not perform TCO.

The Ruby Language Specification doesn't say anything about TCO. It doesn't say you have to do it, but it also doesn't say you can't do it. You just can't rely on it.

This is unlike Scheme, where the Language Specification requires that all Implementations must perform TCO. But it is also unlike Python, where Guido van Rossum has maid it very clear on multiple occasions (the last time just a couple of days ago) that Python Implementations should not perform TCO.

Yukihiro Matsumoto is sympathetic to TCO, he just doesn't want to force all Implementations to support it. Unfortunately, this means that you cannot rely on TCO, or if you do, your code will no longer be portable to other Ruby Implementations.

So, some Ruby Implementations perform TCO, but most don't. YARV, for example, supports TCO, although (for the moment) you have to explicitly uncomment a line in the source code and recompile the VM, to activate TCO – in future versions it is going to be on by default, after the implementation proves stable. The Parrot Virtual Machine supports TCO natively, therefore Cardinal could quite easily support it, too. The CLR has some support for TCO, which means that IronRuby and Ruby.NET could probably do it. Rubinius could probably do it, too.

But JRuby and XRuby don't support TCO, and they probably won't, unless the JVM itself gains support for TCO. The problem is this: if you want to have a fast implementation, and fast and seamless integration with Java, then you should be stack-compatible with Java and use the JVM's stack as much as possible. You can quite easily implement TCO with trampolines or explicit continuation-passing style, but then you are no longer using the JVM stack, which means that everytime you want to call into Java or call from Java into Ruby, you have to perform some kind of conversion, which is slow. So, XRuby and JRuby chose to go with speed and Java integration over TCO and continuations (which basically have the same problem).

This applies to all implementations of Ruby that want to tightly integrate with some host platform that doesn't support TCO natively. For example, I guess MacRuby is going to have the same problem.