Clojure vs Java, pt. 1 | Hacker News Reader

Clojure vs Java, pt. 1

tech.puredanger.com

59 pointsDanielRibeiro19 comments

Clojure Protocols are really a great way to extend types. I think it's a nice fit with both the functional and dynamic nature of Clojure, and it is carefully designed to allow "open" types while completely avoiding monkey-patching. It just seems like the perfect balance, even if it's hard to understand at first. I think it might even lead to "better" OOP in Clojure than in Java.

I would love to have this in Ruby, so that classes could be extended but not in surprising ways.

Exactly. Funny thing. Better OOP in a functional language than in a OOP-language.

But i don't think it's hard to understand. I've seen a lot of good explanations in presentations and articles. Also the concept is really easy.

I keep hearing that current JVM implementation prevents tail call optimizations [1]. Is it still the case nowadays? Kind of curious on that since there seem to be lots of folks on HN use Clojure in production systems.

[1] http://stackoverflow.com/questions/105834/does-the-jvm-preve...

You use recur in clojure for tail optimization. Don't think the jvm has tail optimization support currently.

Exactly, it doesn't. Only self-recursion with recur is tail optimized by the JVM

recur isn't tail-optimized by the JVM. It's converted by the Clojure compiler into a loop.

True, my bad

To add to this, Clojure also has "trampoline" for TCO in mutually recursive functions.

Is recur kind of workaround? Any performance penalty?

Please bear with me as I'm not familiar with Lisp family languages. I'm interested in ML languages. There is a project to port OCaml to JVM, but I don't see any sign of progress, so I thought it may be something on JVM to hold them back.

Recur is essentially just a loop - it doesn't use the stack at all.

Recur relies on AST rewriting to turn self-recursion into a loop. The JVM doesn't play a part in this. My impression is that this is 50/50 language design (recur throws errors on non-tail calls) and compromise for the JVM as a platform.

OCaml on the JVM would have the same problems, if not worse. Eliminating tail recursion is essential in the ML family, and doing that while sanely supporting Java interop seems to be difficult.

I don't really keep up with it much, but I remember proposals to give some bytecode operations that would help here in the next Java version. Not sure if any of them made it past that stage.

I actually like the 'recur' better than implicit tail-call optimization. It does what it says on the tin. If you don't use recur, you get unoptimized recursion, and more importantly: where you can't make a function tail-recursive, you can't use 'recur', because it is a compiler error.

A longer answer, because there seems to be a lot of confusion here.

The JVM security model allows a function to goto anywhere inside the current function, but does not allow a goto to outside the current function.

This means that JVM based languages can do tail call optimizations in self-recursive functions, but can't TCO in mutually recursive functions.

With Clojure, Rich Hickey thought there was some accidental complexity here, so he added the recur special form. Recur guarantees the call is in the tail position, and throws an exception if it isnt. Recur can go to the top of the function, or an enclosing loop block inside the function.

To do mutual recursion in Clojure, you use a trampoline. That looks like

    (declare even? odd?)

    (defn even? [x]
      (if (= x 0)
        true
        #(odd? (dec x)))

    (defn odd? [x]
      (if (= x 1)
        true
        #(even? (dec x)))

    (trampoline even? 100)

What we're doing here is defining two mutually recursive functions, but instead of calling the other directly, we return a function of no arguments that calls the other function. Then call trampoline, passing the function and any arguments. Not the most elegant solution, but it works.

Clojure provides three solutions to work around the lack of TCO - recur, trampoline, and lazy sequences. In my experience these provide performance equal to recursive Racket code and I've been able to port Scheme code that expected TCO with little trouble to Clojure.

Yes, it's still the case that language implementors have to work around the JVM's lack of support for tail call optimization. It's not a big deal for most apps. I leave TCO turned off with Kawa Scheme not because it's a performance problem, but just because it's the default and I don't usually need it. The only code I have compiled with TCO enabled was for maze generation, and I don't think it slowed it down much: http://brlewis.com/amaze.brl

Is it possible to implement the same protocol for a particular type in multiple ways?

No, as a reimplementation will effectively replace the prior implementation. But I think you should ask whether that question makes sense.

If you're dispatching based on something other than just type (some information that would allow you to choose one of multiple implementations) than you should probably either a) put that logic inside the protocol implementation for that type or b) use multimethods which let you use an arbitrary dispatch function that can take both type and other information into account. If you choose a), you could actually layer another protocol or other dispatch mechanism under the initial call if you need to.

Imagine I have a JSONfiable protocol and I have a generic function to traverse a structure that uses the protocol to make a JSON document. It would be nice if the caller had a way to change the format.

You can change the implementation of the protocol, but then it's changed (in that it replaces the prior version). When you see something of type X, the protocol says what to do when you encounter a function on X. If you want more than one kind of thing to happen, then you shouldn't use a protocol.

For instance, it would be perfectly feasible to use a multimethod that dispatches on both type and output format. Or a protocol that dispatches on type, then dispatches a different function on format.

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