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  • Macros's that define macros

    - by David Thornley
    Does anyone know how to pull off something like this... I have alot of repetitive macros as : - #define MYMACRO1(x) Do1(x) #define MYMACRO2(x,y) Do2(x, y) #define MYNEXTMACRO1(x) Do1(x) #define MYNEXTMACRO2(x,y) Do2(x, y) The code above works fine, but I want to write a macro that creates macros (a meta macro). For example: - #define MYMETAMACRO(name) \ #define #name1(x) Do1(x) \ #define #name2(x,y) Do2(x, y) \ Such that I can do : - MYMETAMACRO(MYMACRO); MYMETAMACRO(MYNEXTMACRO); and then : - MYMACRO1(2); MYMACRO2(2,3); MYNEXTMACRO1(4); MYNEXTMACRO2(4, 5); The preprocessor bombs out at the #define as it thinks it is a missing parameter of the macro.

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  • How do I implement a collection in Scala 2.8?

    - by Simon Reinhardt
    In trying to write an API I'm struggling with Scala's collections in 2.8(.0-beta1). Basically what I need is to write something that: adds functionality to immutable sets of a certain type where all methods like filter and map return a collection of the same type without having to override everything (which is why I went for 2.8 in the first place) where all collections you gain through those methods are constructed with the same parameters the original collection had (similar to how SortedSet hands through an ordering via implicits) which is still a trait in itself, independent of any set implementations. Additionally I want to define a default implementation, for example based on a HashSet. The companion object of the trait might use this default implementation. I'm not sure yet if I need the full power of builder factories to map my collection type to other collection types. I read the paper on the redesign of the collections API but it seems like things have changed a bit since then and I'm missing some details in there. I've also digged through the collections source code but I'm not sure it's very consistent yet. Ideally what I'd like to see is either a hands-on tutorial that tells me step-by-step just the bits that I need or an extensive description of all the details so I can judge myself which bits I need. I liked the chapter on object equality in "Programming in Scala". :-) But I appreciate any pointers to documentation or examples that help me understand the new collections design better.

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  • Clojure Protocols vs Scala Structural Types

    - by Vasil Remeniuk
    After watching the interview with Rich Hickey on Protocols in Clojure 1.2, and knowing very little about Clojure, I have some questions on Clojure Protocols: Are they intended to do the same thing as Structural Types in Scala? What benefits do Protocols have over Structural Types (performance, flexibility, code clarity, etc.)? Are they implemented through reflections? Questions on interoperability with Scala: Can Protocols be used instead of Structural Types in Scala? Can they be extended (if 'extension' term can be applied to Protocols) in Scala?

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  • Scala command line parameters in eclipse?

    - by Mitch Blevins
    Scala includes the continuations plugin now (yay), but must be enabled by passing "-P:continuations:enable" to the scala compiler. Is there a way to pass arbitrary arguments to scalac for the eclipse scala plugin? From: http://permalink.gmane.org/gmane.comp.lang.scala/19439 the plugin is loaded by default, but it must be enabled by the command line argument -P:continuations:enable

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  • Scala actors: receive vs react

    - by jqno
    Let me first say that I have quite a lot of Java experience, but have only recently become interested in functional languages. Recently I've started looking at Scala, which seems like a very nice language. However, I've been reading about Scala's Actor framework in Programming in Scala, and there's one thing I don't understand. In chapter 30.4 it says that using react instead of receive makes it possible to re-use threads, which is good for performance, since threads are expensive in the JVM. Does this mean that, as long as I remember to call react instead of receive, I can start as many Actors as I like? Before discovering Scala, I've been playing with Erlang, and the author of Programming Erlang boasts about spawning over 200,000 processes without breaking a sweat. I'd hate to do that with Java threads. What kind of limits am I looking at in Scala as compared to Erlang (and Java)? Also, how does this thread re-use work in Scala? Let's assume, for simplicity, that I have only one thread. Will all the actors that I start run sequentially in this thread, or will some sort of task-switching take place? For example, if I start two actors that ping-pong messages to each other, will I risk deadlock if they're started in the same thread? According to Programming in Scala, writing actors to use react is more difficult than with receive. This sounds plausible, since react doesn't return. However, the book goes on to show how you can put a react inside a loop using Actor.loop. As a result, you get loop { react { ... } } which, to me, seems pretty similar to while (true) { receive { ... } } which is used earlier in the book. Still, the book says that "in practice, programs will need at least a few receive's". So what am I missing here? What can receive do that react cannot, besides return? And why do I care? Finally, coming to the core of what I don't understand: the book keeps mentioning how using react makes it possible to discard the call stack to re-use the thread. How does that work? Why is it necessary to discard the call stack? And why can the call stack be discarded when a function terminates by throwing an exception (react), but not when it terminates by returning (receive)? I have the impression that Programming in Scala has been glossing over some of the key issues here, which is a shame, because otherwise it's a truly excellent book.

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  • Using Scala array from java

    - by Grégoire
    Hi I'm trying to use some library code written in scala from a java program. I have a function that returns an Array (a scala Array) and I thought it would be possible to do Tree[] = ScalaObject.myScalaFunction() But the I get this error : [error] found : scala.runtime.BoxedArray [error] required: org.grammaticalframework.Trees.Absyn.Tree[] What is the correct way to use a scala array in java ?

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  • Samples of Scala and Java code where Scala code looks simpler/has fewer lines?

    - by Roman
    I need some code samples (and I also really curious about them) of Scala and Java code which show that Scala code is more simple and concise then code written in Java (of course both samples should solve the same problem). If there is only Scala sample with comment like "this is abstract factory in Scala, in Java it will look much more cumbersome" then this is also acceptable. Thanks!

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  • Parser that accepts Scala Identifiers?

    - by Mirko Stocker
    I was wondering whether the standard Scala parser combinators contain a parser that accepts the same identifiers that the Scala language itself also accepts (as specified in the Scala Language Specification, Section 1.1). The StdTokenParsers trait has an ident parser, but it rejects identifiers like empty_?. (If there is indeed no such parser, I could also just instantiate the Scala parser itself, but that wouldn't be as lightweight anymore.)

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  • Samples of Scala and Java code where Scala code looks simpler/has less lines?

    - by Roman
    I need some code samples (and I also really curious about them) of Scala and Java code which show that Scala code is more simple and concise then code written in Java (of course both samples should solve the same problem). If there is only Scala sample with comment like "this is abstract factory in Scala, in Java it will look much more cumbersome" then this is also acceptable. Thanks!

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  • In Scala, when is (A,A)=>R not equivalent to Function2 ?

    - by Alex R
    I'm trying to define and use my own foreach function. This is in the middle of a larger block of code. But the essence of the error I'm getting is this: test_2.scala:32: error: type mismatch; found : (A, A) = Unit required: Function2 $amount.foreach( (k:A,v:A) = { How is this error conceivably possible? Isn't (A, A) => Unit always a subtype of Function2 regardless of what else might be going on in the code?

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  • How to write Tetris in Scala? (code review)

    - by eed3si9n
    Today's the 25th birthday of Tetris. I believe writing Tetris clone is one of the best ways to familiarize oneself to a new language or a platform. It's not completely trivial and it lends itself well to learning language specific constructs like iterators and closures. I've been hearing about Scala, and finally decided to read some docs and write a Tetris clone. So, this is my first Scala code. I did try to use functional constructs, but am sure there are lots of things I can improve to do it more Scala way. Please give me suggestions using comment. Also other submissions of Tetris clone in Scala are welcome too. I'm aware that the actual question itself is somewhat subjective, but I think this is of some value since others can use this as example (or anti-example) code. Edit: Let me rephrase the question. What can I do to make the code more Scala-ish?

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  • How to use @PersistentCapable annotation in Scala 2.8

    - by Gero
    Hi, I'm switching from Scala 2.7.7 to Scala 2.8.0RC3 and now a few of my classes don't compile anymore. The problem is in the @PersistentCapable annotation: import javax.jdo.annotations._ import java.util.Date @PersistenceCapable{identityType=IdentityType.APPLICATION} class Counter(dt: Date, cName: String, vl: int) { <.. snip ..> } This code results in the following compilation errors: [ERROR] /Users/gero/prive/kiva/kivanotify-gae/src/main/scala/net/vermaas/kivanotify/model/LoanProcessed.scala:7: error: expected start of definition [INFO] @PersistenceCapable{val identityType = IdentityType.APPLICATION} I already tried a couple of variations, did some Googling but without luck. Any ideas on how I can use the @PersistentCapable annotation with Scala 2.8.0 RC3? Thanks, Gero

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  • Catch all exceptions in Scala 2.8 RC1

    - by Michel Krämer
    I have the following dummy Scala code in the file test.scala: class Transaction { def begin() {} def commit() {} def rollback() {} } object Test extends Application { def doSomething() {} val t = new Transaction() t.begin() try { doSomething() t.commit() } catch { case _ => t.rollback() } } If I compile this on Scala 2.8 RC1 with scalac -Xstrict-warnings test.scala I'll get the following warning: test.scala:16: warning: catch clause swallows everything: not advised. case _ => t.rollback() ^ one warning found So, if catch-all expressions are not advised, how am I supposed to implement such a pattern instead? And apart from that why are such expressions not advised anyhow?

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  • Setting Scala "Platform" for NetBeans 6.8 on Ubuntu Lucid Lynx

    - by David
    I'm trying to use NetBeans 6.8 with Scala, and it can't find the "Scala Platform" (whatever that is supposed to be). I'm using Ubuntu Lucid Lynx (fully updated). The libraries are in /usr/share/java, the binaries in /usr/bin, the docs in /usr/share/doc/scala-doc/, and the sources are uninstalled. I think that NetBeans is looking for Scala to be in one single directory (with bin, lib, etc.). I created /usr/share/scala and placed links to the other directories, but it still didn't quite make it. Can someone straighten me out? (And why is it a "platform"? More needless jargon...)

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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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  • makefile: how to call macros in macros

    - by Matthias Günther
    Hello, I have the following macros in my make file: pdf: // do something clean: // just another fancy thing No I want to declare a macro all: which include (or call) the makros above. The following thing doesn't work: all: pdf: clean: I don't want to repeat the code from pdf: and clean: in order not to rebel against DRY principle. Thanks for your help.

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