Less sarcastically, the mere existence of the builder pattern in Java, which undermines referential transparency to its core, is proof that the design of Java as both a language and community is strongly biased against a functional, referentially transparent, mathematical approach to programming, even if such a style were theoretically possible[1].
[2] Scala and Clojure being proof that it is still possible (though only to a lesser extent - tail call elimination and corecursion being an example where it fails completely).
> [2] Scala and Clojure being proof that it is still possible (though only to a lesser extent - tail call elimination and corecursion being an example where it fails completely).
In clojure, loop..recur and trampoline does the job. Tail recursion is not automatic, but still, it's there and quite straight forward to use with the added benefit of recur cribbing if your call isn't a tail call.
This has been discussed to death already, so without opening Pandora's box too much: (1) If a compiler can't automatically eliminate tail calls, that's either a design flaw or a bug, and (2) If I wanted to write a loop, I wouldn't be asking for recursion - even if they're executed in the same way by the machine, I'm expressing something different by writing my code that way.
> If a compiler can't automatically eliminate tail calls, that's either a design flaw or a bug
You may know this already, but my understanding is that Clojure opts to not automatically eliminate tail calls in order to use Java calling conventions, which I imagine simplifies or eases interop. I'm not defending this decision, as I don't have much of an opinion on it, but to call it a design flaw or a bug seems a bit disingenuous as there is definitely a tradeoff to be made.
I don't know why Clojure does not do TCO, but I would be surprised if the Java VM has something to do with it. I mean, why? TCO is just an AST transformation. Takes a function, returns a function and a for loop. Yeah, yeah, the easy implementation needs goto, but it's not necessary. AFAICT any tail-recursive function can be implemented as a for loop plus continue, break and return.
Now, while Clojure just not implementing automatic TCO would not be a bug or a design flaw per se -- clisp does fairly well without it, because the developers just didn't bother -- if implementing such an AST transformation in the Clojure compiler is impossible, then the compiler's design is faulty. It's like programming FizzBuzz so that you can't later modify it to add a case for multiples of 7. OK, yeah, TCO without goto is not such a piece of cake, but you get the idea.
And I don't think it's a trade-off in the least. If so, what are Clojure programmers winning now? The glorious overhead of thousands of function calls? Those tasty "stack overflow" back-traces, with thousands of calls to the same function?
I did a little homework before posting the comment you're replying to, and found a few sources stating that Clojure lacks general tail call optimization due to limitations in the JVM. I'll share what I dug up with you, although to be honest I'm almost just making an appeal to authority (actually to several authorities!), because I haven't actually read the JVM spec.
Rich Hickey wrote:
"No language that uses the JVM stack and calling convention (e.g. Clojure and Scala) can do TCO since it would require either stack manipulation capabilities not offered in the bytecode spec or direct support in the JVM. The latter is the best hope for functional languages on the JVM, but I'm not sure is a priority for Sun as tail-calls are not idiomatic for JRuby/Jython/ Groovy." [0]
I thought that there are implementations of Scheme and other languages on the JVM that do tail call optimization, but I assumed they utilised some kind of virtual stack. Jörg W Mittag on StackOverflow confirmed this:
"Nah, TCO [on the JVM] is easy. Seph does it, Erjang does it, Kawa and all the other Scheme implementations on the JVM do it. The JVM has Exceptions, which are basically the same as GOTO, which can be used to implement TCO. Or you use trampolines. Or you don't use the JVM call stack at all and just implement your own. The reason why Clojure and Scala only provide limited TCO (basically, only tail recursion is optimized) is because they want to use the JVM call stack for interoperability and performance reasons. As Rich Hickey, Clojure's designer said: Interop, speed, TCO -- Pick two." [1]
James Iry wrote the following on LtU, which explains why even though "Real instruction sets (or C) don't 'support' proper tail recursion either", the fact that the JVM doesn't is an issue for Clojure (and Scala):
"Native instruction sets often let you do whatever you want with the stack. C doesn't in the ANSI standard, but you can do it with a bit of assembly. .NET IL has an explicit instruction for tail calls. The JVM, on the other hand, is very strict about how you use its stack and has no tail call instruction." [2]
During my Googling, I also found this[3], which is pretty interesting!
"CTCO works by applying a first-order one-pass CPS algorithm (via Danvy 2007), then transforming the code to return thunks, and finally creating a custom trampoline to be used when the code is executed. Thanks to the properties of the CPS transformation, CTCO will make all function calls into tail calls, thereby even making non-tail code compiled by CTCO use constant space."
This actually sounds not far from what you suggested! Although it does seem to be a bit more than "just an AST transformation". I haven't read through it, or read the referenced paper [4] it's a bit over my head. You should check it out and report back! I've bookmarked it for now.
Finally, you wrote:
"And I don't think it's a trade-off in the least. If so, what are Clojure programmers winning now? The glorious overhead of thousands of function calls? Those tasty 'stack overflow' back-traces, with thousands of calls to the same function?"
Do you still think it's not a trade-off? Unless that last link I found is a magic bullet that makes TCO fast without interfering with Java interop (or the performance of Java interop) then I really don't think it's fair to say that it's not a design trade-off. Rich Hickey (and Martin Odersky) obviously made a conscious decision about this, and you're welcome to disagree with them, but it's not clear cut.
What Clojure programmers are winning now (by being hosted on the JVM) is a lot, I think. Easy interop is one of the pillars of Clojure that has contributed heavily to it picking up steam. I think the same applies to Scala. I said earlier that I don't have an opinion, and I guess it's only true that I don't have a strong opinion. If I had to pick, I think I'd pick interop.
Sorry for the long comment but I'd already done the research, I figured I might as well share it, although there really already has been a great deal said about this.
Less sarcastically, the mere existence of the builder pattern in Java, which undermines referential transparency to its core, is proof that the design of Java as both a language and community is strongly biased against a functional, referentially transparent, mathematical approach to programming, even if such a style were theoretically possible[1].
[1] eg. com.sun.java.swing.plaf.nimbus.InternalFrameInternalFrameTitlePaneInternalFrameTitlePaneMaximizeButtonWindowNotFocusedState
[2] Scala and Clojure being proof that it is still possible (though only to a lesser extent - tail call elimination and corecursion being an example where it fails completely).