7 ms·

How Scala Compiles to Java

73 points by fogus 16y ago

paulitex 16y ago
Very cool, but as someone unfamiliar with scala (and familiar with java) I would really have liked to see java code for those 6 class files. Obviously they would have to have been approximations, but seeing the 6 raw java source files vs. the 1 scala (and the same .class outputs) I feel would have made a very nice comparison. It also would do away with any ambiguity in the verbal description of the congruencies between the two.
- jorgeortiz85 16y ago
  This is approximately how the first code block in the OP would look like in Java 5: import scala.Function1; import scala.List; import scala.runtime.RichInt; class App$ { public static final App$ MODULE$ = new App$(); public boolean isEven(int i) { return i % 2 == 0; } public boolean isOdd(int i) { return i % 2 == 1; } public void main(String[] args) { final List<Integer> n = (new RichInt(1)).to(10).toList(); n.filter(new Function1<Integer, Boolean>() { public Boolean apply(final Integer x) { return isEven(x); } }).flatMap(new Function1<Integer, List<Integer>>() { public List<Integer> apply(final Integer i) { return n.filter(new Function1<Integer, Boolean>() { public Boolean apply(final Integer x) { return isOdd(x); } }).map(new Function1<Integer, Integer>() { public Integer apply(final Integer j) { return i * j; } }); } }); } } class App { public static boolean isEven(int i) { return App$.MODULE$.isEven(i); } public static boolean isOdd(int i) { return App$.MODULE$.isOdd(i); } public static void main(String[] args) { App$.MODULE$.main(args); } } Note that, although anonymous inner classes are valid syntax in Java 5, even in Java 5 they are compiled down to separate class files. Hence 2 classes + 4 anonymous inner classes = 6 class files total. (The code is only approximate. I haven't tried to compile it, so apologies for any errors.)
pacoverdi 16y ago
Interesting post. Another post that I'd like to read would be: How Many Objects Does Scala Allocate (while executing some random code). In my Java shop, we try to write ultra-efficient code. Avoiding the creation of numerous small objects in critical sections is one of the best ways to achieve that. I'd like to start a couple projects in scala or clojure but I'm afraid that the simplicity and expressiveness that these languages bring come at the cost of performance (or at least on control on what the JVM does).
- jongraehl 16y ago
  At least you can write java-style code (or call java seamlessly) for whatever is performance-critical. I agree that idiomatic Scala can be too slow in some cases (it should be faster due to static typing than Clojure wherever methods aren't generic, or are specialized).
  - loewenskind 16y ago
    >should be faster due to static typing than Clojure Uh, why? Are all object types known at runtime in Clojure? I bet they are. And this is exactly what Hotspot was designed to make fast [1]. [1] http://www.strongtalk.org http://www.strongtalk.org
    - kikibobo69 16y ago
      Unfortunately "as good or better" performance is not realized yet, at least not at theThe Computer Language Benchmarks Game[1]. More code, more time, more memory required by Clojure than Scala, to solve the same problems. However the Clojure implementations on the site are still quite young, so some improvement may be possible. [1] http://shootout.alioth.debian.org/u32/benchmark.php?test=all&lang=clojure&lang2=scala http://shootout.alioth.debian.org/u32/benchmark.php?test=all...
      - loewenskind 16y ago
        My point was that it isn't the typing that makes it fast. Hotspot is based on the idea that the type is known at runtime. The Shootout probably wont show good results for this because it takes a little bit for Hotspot to get the most out of it. If you look at the Mandelbrot demo in Strongtalk it takes about 3 full runs to get the best speed.

73 points by fogus 16y ago

Very cool, but as someone unfamiliar with scala (and familiar with java) I would really have liked to see java code for those 6 class files. Obviously they would have to have been approximations, but seeing the 6 raw java source files vs. the 1 scala (and the same .class outputs) I feel would have made a very nice comparison. It also would do away with any ambiguity in the verbal description of the congruencies between the two.

jorgeortiz85 16y ago
This is approximately how the first code block in the OP would look like in Java 5: import scala.Function1; import scala.List; import scala.runtime.RichInt; class App$ { public static final App$ MODULE$ = new App$(); public boolean isEven(int i) { return i % 2 == 0; } public boolean isOdd(int i) { return i % 2 == 1; } public void main(String[] args) { final List<Integer> n = (new RichInt(1)).to(10).toList(); n.filter(new Function1<Integer, Boolean>() { public Boolean apply(final Integer x) { return isEven(x); } }).flatMap(new Function1<Integer, List<Integer>>() { public List<Integer> apply(final Integer i) { return n.filter(new Function1<Integer, Boolean>() { public Boolean apply(final Integer x) { return isOdd(x); } }).map(new Function1<Integer, Integer>() { public Integer apply(final Integer j) { return i * j; } }); } }); } } class App { public static boolean isEven(int i) { return App$.MODULE$.isEven(i); } public static boolean isOdd(int i) { return App$.MODULE$.isOdd(i); } public static void main(String[] args) { App$.MODULE$.main(args); } } Note that, although anonymous inner classes are valid syntax in Java 5, even in Java 5 they are compiled down to separate class files. Hence 2 classes + 4 anonymous inner classes = 6 class files total. (The code is only approximate. I haven't tried to compile it, so apologies for any errors.)

pacoverdi 16y ago

Interesting post. Another post that I'd like to read would be: How Many Objects Does Scala Allocate (while executing some random code). In my Java shop, we try to write ultra-efficient code. Avoiding the creation of numerous small objects in critical sections is one of the best ways to achieve that. I'd like to start a couple projects in scala or clojure but I'm afraid that the simplicity and expressiveness that these languages bring come at the cost of performance (or at least on control on what the JVM does).

jongraehl 16y ago
At least you can write java-style code (or call java seamlessly) for whatever is performance-critical. I agree that idiomatic Scala can be too slow in some cases (it should be faster due to static typing than Clojure wherever methods aren't generic, or are specialized).
- loewenskind 16y ago
  >should be faster due to static typing than Clojure Uh, why? Are all object types known at runtime in Clojure? I bet they are. And this is exactly what Hotspot was designed to make fast [1]. [1] http://www.strongtalk.org http://www.strongtalk.org
  - kikibobo69 16y ago
    Unfortunately "as good or better" performance is not realized yet, at least not at theThe Computer Language Benchmarks Game[1]. More code, more time, more memory required by Clojure than Scala, to solve the same problems. However the Clojure implementations on the site are still quite young, so some improvement may be possible. [1] http://shootout.alioth.debian.org/u32/benchmark.php?test=all&lang=clojure&lang2=scala http://shootout.alioth.debian.org/u32/benchmark.php?test=all...
    - loewenskind 16y ago
      My point was that it isn't the typing that makes it fast. Hotspot is based on the idea that the type is known at runtime. The Shootout probably wont show good results for this because it takes a little bit for Hotspot to get the most out of it. If you look at the Mandelbrot demo in Strongtalk it takes about 3 full runs to get the best speed.