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  • How well does Scala Perform Comapred to Java?

    - by Teja Kantamneni
    The Question actually says it all. The reason behind this question is I am about to start a small side project and want to do it in Scala. I am learning scala for the past one month and now I am comfortable working with it. The scala compiler itself is pretty slow (unless you use fsc). So how well does it perform on JVM? I previously worked on groovy and I had seen sometimes over performed than java. My Question is how well scala perform on JVM compared to Java. I know scala has some very good features(FP, dynamic lang, statically typed...) but end of the day we need the performance...

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  • Print the next X number of lines in Scala

    - by soulesschild
    Trying to learn Scala using the Programming in Scala book and they have a very basic example for reading lines from a file. I'm trying to expand on it and read a file line by line, look for a certain phrase, then print the next 6 lines following that line if it finds the line. I can write the script easily enough in something like java or Perl but I have no idea how to do it in Scala (probably because I'm not very familiar with the language yet...) Here's the semi adapted sample code from the Programming in Scala book, import scala.io.Source if(args.length>0) { val lines = Source.fromFile(args(0)).getLines().toList for(line<-lines) { if(line.contains("secretPhrase")) { println(line) //How to get the next lines here? } } } else Console.err.println("Pleaseenterfilename")

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  • Common practice for higher-order-polymorphism in scala

    - by raichoo
    Hi, I'm trying to grasp higher-order-polymophism in scala by implementing a very basic interface that describes a monad but I come across a problem that I don't really understand. I implemented the same with C++ and the code looks like this: #include <iostream> template <typename T> class Value { private: T value; public: Value(const T& t) { this->value = t; } T get() { return this->value; } }; template < template <typename> class Container > class Monad { public: template <typename A> Container<A> pure(const A& a); }; template <template <typename> class Container> template <typename A> Container<A> Monad<Container>::pure(const A& a) { return Container<A>(a); } int main() { Monad<Value> m; std::cout << m.pure(1).get() << std::endl; return 0; } When trying to do the same with scala I fail: class Value[T](val value: T) class Monad[Container[T]] { def pure[A](a: A): Container[A] = Container[A](a) } object Main { def main(args: Array[String]): Unit = { val m = new Monad[Value] m.pure(1) } } The compiler complains about: [raichoo@lain:Scala]:434> scalac highorder.scala highorder.scala:5: error: not found: value Container Container[A](a) ^ one error found What am I doing wrong here? There seems to be a fundamental concept I don't seem to understand about scala typeconstructors. Regards, raichoo

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  • Why does this explicit call of a Scala method allow it to be implicitly resolved?

    - by Matt R
    Why does this code fail to compile, but compiles successfully when I uncomment the indicated line? (I'm using Scala 2.8 nightly). It seems that explicitly calling string2Wrapper allows it to be used implicitly from that point on. class A { import Implicits.string2Wrapper def foo() { //string2Wrapper("A") ==> "B" // <-- uncomment } def bar() { "A" ==> "B" "B" ==> "C" "C" ==> "D" } object Implicits { implicit def string2Wrapper(s: String) = new Wrapper(s) class Wrapper(s: String) { def ==>(s2: String) {} } } }

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  • Are Scala "continuations" just a funky syntax for defining and using Callback Functions?

    - by Alex R
    And I mean that in the same sense that a C/Java for is just a funky syntax for a while loop. I still remember when first learning about the for loop in C, the mental effort that had to go into understanding the execution sequence of the three control expressions relative to the loop statement. Seems to me the same sort of effort has to be applied to understand Continuations (in Scala and I guess probably other languages). And then there's the obvious follow-up question... if so, then what's the point? It seems like a lot of pain (language complexity, programmer errors, unreadable programs, etc) for no gain.

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  • What is the proper way to code a read-while loop in Scala?

    - by ARKBAN
    What is the "proper" of writing the standard read-while loop in Scala? By proper I mean written in a Scala-like way as opposed to a Java-like way. Here is the code I have in Java: MessageDigest md = MessageDigest.getInstance( "MD5" ); InputStream input = new FileInputStream( "file" ); byte[] buffer = new byte[1024]; int readLen; while( ( readLen = input.read( buffer ) ) != -1 ) md.update( buffer, 0, readLen ); return md.digest(); Here is the code I have in Scala: val md = MessageDigest.getInstance( hashInfo.algorithm ) val input = new FileInputStream( "file" ) val buffer = new Array[ Byte ]( 1024 ) var readLen = 0 while( readLen != -1 ) { readLen = input.read( buffer ) if( readLen != -1 ) md.update( buffer, 0, readLen ) } md.digest The Scala code is correct and works, but feels very un-Scala-ish. For one it is a literal translation of the Java code, taking advantage of none of the advantages of Scala. Further it is actually longer than the Java code! I really feel like I'm missing something, but I can't figure out what. I'm fairly new to Scala, and so I'm asking the question to avoid falling into the pitfall of writing Java-style code in Scala. I'm more interested in the Scala way to solve this kind of problem than in any specific helper method that might be provided by the Scala API to hash a file. (I apologize in advance for my ad hoc Scala adjectives throughout this question.)

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  • How to define a ternary operator in Scala which preserves leading tokens?

    - by Alex R
    I'm writing a code generator which produces Scala output. I need to emulate a ternary operator in such a way that the tokens leading up to '?' remain intact. e.g. convert the expression c ? p : q to c something. The simple if(c) p else q fails my criteria, as it requires putting if( before c. My first attempt (still using c/p/q as above) is c match { case(true) = p; case _ = q } another option I found was: class ternary(val g: Boolean = Any) { def |: (b:Boolean) = g(b) } implicit def autoTernary (g: Boolean = Any): ternary = new ternary(g) which allows me to write: c |: { b: Boolean = if(b) p else q } I like the overall look of the second option, but is there a way to make it less verbose? Thanks

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  • Java Champion Dick Wall Explores the Virtues of Scala (otn interview)

    - by Janice J. Heiss
    In a new interview up on otn/java, titled “Java Champion Dick Wall on the Virtues of Scala (Part 2),” Dick Wall explains why, after a long career in programming exploring Lisp, C, C++, Python, and Java, he has finally settled on Scala as his language of choice. From the interview: “I was always on the lookout for a language that would give me both Python-like productivity and simplicity for just writing something and quickly having it work and that also offers strong performance, toolability, and type safety (all of which I like in Java). Scala is simply the first language that offers all those features in a package that suits me. Programming in Scala feels like programming in Python (if you can think it, you can do it), but with the benefit of having a compiler looking over your shoulder and telling you that you have the wrong type here or the wrong method name there.The final ‘aha!’ moment came about a year and a half ago. I had a quick task to complete, and I started writing it in Python (as I have for many years) but then realized that I could probably write it just as fast in Scala. I tried, and indeed I managed to write it just about as fast.”Wall makes the remarkable claim that once Java developers have learned to work in Scala, when they work on large projects, they typically find themselves more productive than they are in Java. “Of course,” he points out, “people are always going to argue about these claims, but I can put my hand over my heart and say that I am much more productive in Scala than I was in Java, and I see no reason why the many people I know using Scala wouldn’t say the same without some reason.”Read the interview here.

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  • Why does Scala apply thunks automatically, sometimes?

    - by Anonymouse
    At just after 2:40 in ShadowofCatron's Scala Tutorial 3 video, it's pointed out that the parentheses following the name of a thunk are optional. "Buh?" said my functional programming brain, since the value of a function and the value it evaluates to when applied are completely different things. So I wrote the following to try this out. My thought process is described in the comments. object Main { var counter: Int = 10 def f(): Int = { counter = counter + 1; counter } def runThunk(t: () => Int): Int = { t() } def main(args: Array[String]): Unit = { val a = f() // I expect this to mean "apply f to no args" println(a) // and apparently it does val b = f // I expect this to mean "the value f", a function value println(b) // but it's the value it evaluates to when applied to no args println(b) // and the evaluation happens immediately, not in the call runThunk(b) // This is an error: it's not println doing something funny runThunk(f) // Not an error: seems to be val doing something funny } }   To be clear about the problem, this Scheme program (and the console dump which follows) shows what I expected the Scala program to do. (define counter (list 10)) (define f (lambda () (set-car! counter (+ (car counter) 1)) (car counter))) (define runThunk (lambda (t) (t))) (define main (lambda args (let ((a (f)) (b f)) (display a) (newline) (display b) (newline) (display b) (newline) (runThunk b) (runThunk f)))) > (main) 11 #<procedure:f> #<procedure:f> 13   After coming to this site to ask about this, I came across this answer which told me how to fix the above Scala program: val b = f _ // Hey Scala, I mean f, not f() But the underscore 'hint' is only needed sometimes. When I call runThunk(f), no hint is required. But when I 'alias' f to b with a val then apply it, it doesn't work: the evaluation happens in the val; and even lazy val works this way, so it's not the point of evaluation causing this behaviour.   That all leaves me with the question: Why does Scala sometimes automatically apply thunks when evaluating them? Is it, as I suspect, type inference? And if so, shouldn't a type system stay out of the language's semantics? Is this a good idea? Do Scala programmers apply thunks rather than refer to their values so much more often that making the parens optional is better overall? Examples written using Scala 2.8.0RC3, DrScheme 4.0.1 in R5RS.

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  • Advantages of Scala vs. Groovy with JAVA EE 6 Applications.

    - by JAVA EE Wannabe
    Please let me first emphasize that I am not looking for flare wars. I just want advices from people who have real experiences. I started learning JAVA EE 6 as real newbie and am having had time choosing what tools to use. First problem is what is the advantage of using Scala vs. Groovy with Java EE 6 apps over Java? I've seen on some blogs people mentioning you gonna write less code but as a newbie I don't know what other advantages and disadvantages are there. Second problem is Netbeans 6.9 or Helios 3.6.1? I realized that with eclipse I can easily mix EE 6 applications with Groovy or Scala without any problems (I only did this by displaying a String message from Scala and Groovy classes.). With Netbeans the only I can think of is having separate Java project libraries and using the jars in my web app. But also realize to the extent of my little knowledge Netbeans has better support for Java EE 6. Please need your expert advice. Thanks.

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  • Scala for Junior Programmers?

    - by Traldin
    Hi, we are considering Scala for a new Project within our company. We have some Junior Programmers with only PHP knowledge, and we are in doubt that they can handle Scala. What are your opinions? Some say: "Scala is a complicated beast!", some say: "It's easy once you got it." Maybe someone has real-world experience?

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  • process csv in scala

    - by portoalet
    I am using scala 2.7.7, and wanted to parse CSV file and store the data in SQLite database. I ended up using OpenCSV java library to parse the CSV file, and using sqlitejdbc library. Using these java libraries makes my scala code looks almost identical to that of Java code (sans semicolon and with val/var) As I am dealing with java objects, I can't use scala list, map, etc, unless I do scala2java conversion or upgrade to scala 2.8 Is there a way I can simplify my code further using scala bits that I don't know? val filename = "file.csv"; val reader = new CSVReader(new FileReader(filename)) var aLine = new Array[String](10) var lastSymbol = "" while( (aLine = reader.readNext()) != null ) { if( aLine != null ) { val symbol = aLine(0) if( !symbol.equals(lastSymbol)) { try { val rs = stat.executeQuery("select name from sqlite_master where name='" + symbol + "';" ) if( !rs.next() ) { stat.executeUpdate("drop table if exists '" + symbol + "';") stat.executeUpdate("create table '" + symbol + "' (symbol,data,open,high,low,close,vol);") } } catch { case sqle : java.sql.SQLException => println(sqle) } lastSymbol = symbol } val prep = conn.prepareStatement("insert into '" + symbol + "' values (?,?,?,?,?,?,?);") prep.setString(1, aLine(0)) //symbol prep.setString(2, aLine(1)) //date prep.setString(3, aLine(2)) //open prep.setString(4, aLine(3)) //high prep.setString(5, aLine(4)) //low prep.setString(6, aLine(5)) //close prep.setString(7, aLine(6)) //vol prep.addBatch() prep.executeBatch() } } conn.close()

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  • noClassDefFoundError using Scala Plugin for Eclipse

    - by Jacob Lyles
    I successfully implemented and ran several Scala tutorials in Eclipse using the Scala plugin. Then suddenly I tried to compile and run an example, and this error came up: Exception in thread "main" java.lang.NoClassDefFoundError: hello/HelloWorld Caused by: java.lang.ClassNotFoundException: hello.HelloWorld at java.net.URLClassLoader$1.run(URLClassLoader.java:200) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:188) at java.lang.ClassLoader.loadClass(ClassLoader.java:315) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:330) at java.lang.ClassLoader.loadClass(ClassLoader.java:250) at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:398) After this point I could no longer run any Scala programs in Eclipse. I tried cleaning and rebuilding my project, closing and reopening my project, and closing and reopening Eclipse. Eclipse version number 3.5.2 and Scala plugin 2.8.0 Here is the original code: package hello object HelloWorld { def main(args: Array[String]){ println("hello world") } }

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  • Scala regex Named Capturing Groups

    - by Brent
    In scala.util.matching.Regex trait MatchData I see that there support for groupnames (Named Capturing Groups) But since Java does not support groupnames until version 7 as I understand it, Scala version 2.8.0.RC4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6. gives me this exception: scala> val pattern = """(?<login>\w+) (?<id>\d+)""".r java.util.regex.PatternSyntaxException: Look-behind group does not have an obvio us maximum length near index 11 (?<login>\w+) (?<id>\d+) ^ at java.util.regex.Pattern.error(Pattern.java:1713) at java.util.regex.Pattern.group0(Pattern.java:2488) at java.util.regex.Pattern.sequence(Pattern.java:1806) at java.util.regex.Pattern.expr(Pattern.java:1752) at java.util.regex.Pattern.compile(Pattern.java:1460) So the question is Named Capturing Groups supported in Scala? If so any examples out there? If not I might look into the Named-Regexp lib from clement.denis.

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  • Scala script to copy files

    - by kulkarni
    I want to copy file a.txt to newDir/ from within a scala script. In java this would be done by creating 2 file streams for the 2 files, reading into buffer from a.txt and writing it to the FileOutputStream of the new file. Is there a better way to achieve this in scala? May be something in scala.tools.nsc.io._. I searched around but could not find much.

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  • Do Scala and Erlang use green threads?

    - by CHAPa
    I've been reading a lot about how Scala and Erlang does lightweight threads and their concurrency model (actors). However, I have my doubts. Do Scala and Erlang use an approach similar to the old thread model used by Java (green threads) ? For example, suppose that there is a machine with 2 cores, so the Scala/Erlang environment will fork one thread per processor? The other threads will be scheduled by user-space (Scala VM / Erlang VM ) environment. Is this correct? Under the hood, how does this really work?

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  • Scala/Erlang use something like greenThread or not ?

    - by CHAPa
    Hi all, Im reading a lot about how scala/Erlang does lightweight threads and your concurrency model ( Actor Model ). Off course, some doubts appear in my head. Scala/Erlang use a approach similar to the old thread model used by java (greenThread) ? for example, suppose that there is a machine with 2 cores, so the scala/erlang environment will fork one thread per processor ? The other threads will be scheduled by user-space( scala VM / erlang vm ) environment. is it correct ? how under the hood that really work ? thanks a lot.

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  • What are the biggest differences between Scala 2.8 and Scala 2.7?

    - by André Laszlo
    I've written a rather large program in Scala 2.75, and now I'm looking forward to version 2.8. But I'm curious about how this big leap in the evolution of Scala will affect me. What will be the biggest differences between these two versions of Scala? And perhaps most importantly: Will I need to rewrite anything? Do I want to rewrite anything just to take advantage of some cool new feature? What exactly are the new features of Scala 2.8 in general?

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  • Common programming mistakes for Scala developers to avoid

    - by jelovirt
    In the spirit of Common programming mistakes for Java developers to avoid? Common programming mistakes for JavaScript developers to avoid? Common programming mistakes for .NET developers to avoid? Common programming mistakes for Haskell developers to avoid? Common programming mistakes for Python developers to avoid? Common Programming Mistakes for Ruby Developers to Avoid Common programming mistakes for PHP developers to avoid? what are some common mistakes made by Scala developers, and how can we avoid them? Also, as the biggest group of new Scala developers come from Java, what specific pitfalls they have to be aware of? For example, one often cited problem Java programmers moving to Scala make is use a procedural approach when a functional one would be more suitable in Scala. What other mistakes e.g. in API design newcomers should try to avoid.

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  • Launch Scala REPL programatically?

    - by David Crawshaw
    I would like to launch a Scala Swing application from the command line, then after the application is started, drop into the Scala REPL to use as a control interface. Ideally I would also like to pre-bind some variable names. Even better would be using a Java2D terminal emulator for the REPL, but I couldn't find anything appropriate. Does the Scala REPL have a public API?

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  • Java->Scala Remove Iterator<T>-Element from a JavaConversions-wrapped Iterable

    - by ifischer
    I have to translate the following code from Java to Scala: for (Iterator<ExceptionQueuedEvent> i = getUnhandledExceptionQueuedEvents().iterator(); i.hasNext();) { ExceptionQueuedEvent event = i.next(); try { //do something } finally { i.remove(); } } I'm using the JavaConversions library to wrap the Iterable. But as i'm not using the original Iterator, i don't know how to remove the current element correctly from the collection the same way as i did in Java: import scala.collection.JavaConversions._ (...) for (val event <- events) { try { //do something } finally { //how can i remove the current event from events? } } Can someone help me? I guess it's easy, but i'm still kinda new to Scala and don't understand what's going on when Scala wraps something of Java.

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  • Scala 2.8: use Java annotation with an array parameter

    - by yournamehere
    I'm trying to implement an JavaEE Session Bean with Scala 2.8. Because it's a Remote Session Bean, i have to annotate it with the following Java Annotation: @Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) public @interface Remote { Class[] value() default {}; } I only found this example for scala 2.7. In Scala 2.7, its possible to define the session bean like this: @Remote {val value = Array(classOf[ITest])} class MyEJB ... How can i use this annotation the same way with Scala 2.8? I already tried many different versions, all resulting in "annotation argument needs to be a constant", "illegal start of simple expression". All of these definitions don't work: @Remote{val value = Array(classOf[PersonScalaEJB])} @Remote(val value = Array(classOf[PersonScalaEJB])) @Remote(Array(classOf[PersonScalaEJB]))

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  • Scala in image procesing

    - by Mayank Sinha
    I am new to scala and keep on researching and compiling programs , But i am more interested in Image processing in scala. Iam doing project in eclipse environment from http://www.scala-lang.org/download/2.11.0-M5.html but i couldn't find any resource to do image processing in scala . please provide me all information i.e How to install image processing package till manuals of the package to read and write image. I have tried Maven ,jmagic,opencv ,javacv etc but couldn't succeeded. Please reply with in hours my job is at stake. Mayank

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  • What ORMs work well with Scala?

    - by Clinton R. Nixon
    I'm about to write a Scala command-line application that relies on a MySQL database. I've been looking around for ORMs, and am having trouble finding one that will work well. The Lift ORM looks nice, but I'm not sure it can be decoupled from the entire Lift web framework. ActiveObjects also looks OK, but the author says that it may not work well with Scala. I'm not coming to Scala from Java, so I don't know all the options. Has anyone used an ORM with Scala, and if so, what did you use and how well did it work?

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