Does the JVM prevent tail call optimizations?

I saw this quote on the question: What is a good functional language on which to build a web service?

Scala in particular doesn't support tail-call elimination except in self-recursive functions, which limits the kinds of composition you can do (this is a fundamental limitation of the JVM).

Is this true? If so, what is it about the JVM that creates this fundamental limitation?

Solution 1:

This post: Recursion or Iteration? might help.

In short, tail call optimization is hard to do in the JVM because of the security model and the need to always have a stack trace available. These requirements could in theory be supported, but it would probably require a new bytecode (see John Rose's informal proposal).

There is also more discussion in Sun bug #4726340, where the evaluation (from 2002) ends:

I believe this could be done nonetheless, but it is not a small task.

Currently, there is some work going on in the Da Vinci Machine project. The tail call subproject's status is listed as "proto 80%"; it is unlikely to make it into Java 7, but I think it has a very good chance at Java 8.

Solution 2:

The fundamental limitation is simply that the JVM does not provide tail calls in its byte code and, consequently, there is no direct way for a language built upon the JVM to provide tail calls itself. There are workarounds that can achieve a similar effect (e.g. trampolining) but they come at the grave cost of awful performance and obfuscating the generated intermediate code which makes a debugger useless.

So the JVM cannot support any production-quality functional programming languages until Sun implement tail calls in the JVM itself. They have been discussing it for years but I doubt they will ever implement tail calls: it will be very difficult because they have prematurely optimized their VM before implementing such basic functionality, and Sun's effort is strongly focused on dynamic languages rather than functional languages.

Hence there is a very strong argument that Scala is not a real functional programming language: these languages have regarded tail calls as an essential feature since Scheme was first introduced over 30 years ago.