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  • I'm having an issue with a scala script using 2.7.7 - Main$$anon$1$$anonfun$1$$anonfun$apply$2

    - by Uruhara747
    This is the code I used from a book... import scala.actors.Actor._ val countActor = actor{ loop { react{ case "how many?" = { println("I've got " + mailboxSize.toString + " messages in my mailbox.") } } } } countActor ! 1 countActor ! 2 countActor ! 3 countActor ! "how many?" countActor ! "how many?" countActor ! 4 countActor ! "how many?" The error java.lang.NoClassDefFoundError: Main$$anon$1$$anonfun$1$$anonfun$apply$2

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  • Why is there an implicit conversion from Float/Double to BigDecimal, but not from String?

    - by soc
    Although the situation of conversion from Doubles to BigDecimals has improved a bit compared to Java scala> new java.math.BigDecimal(0.2) res0: java.math.BigDecimal = 0.20000000000000001110223024625156... scala> BigDecimal(0.2) res1: scala.math.BigDecimal = 0.2 and things like val numbers: List[BigDecimal] = List(1.2, 3.2, 0.7, 0.8, 1.1) work really well, wouldn't it be reasonable to have an implicit conversion like implicit def String2BigDecimal(s: String) = BigDecimal(s) available by default which can convert Strings to BigDecimals like this? val numbers: List[BigDecimal] = List("1.2", "3.2", "0.7", "0.8", "1.1") Or am I missing something and Scala resolved all "problems" of Java with using the BigDecimal constructor with a floating point value instead of a String, and BigDecimal(String) is basically not needed anymore in Scala?

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  • Is there an equivalent in Scala to Python's more general map function?

    - by wheaties
    I know that Scala's Lists have a map implementation with signature (f: (A) => B):List[B] and a foreach implementation with signature (f: (A) => Unit):Unit but I'm looking for something that accepts multiple iterables the same way that the Python map accepts multiple iterables. I'm looking for something with a signature of (f: (A,B) => C, Iterable[A], Iterable[B] ):Iterable[C] or equivalent. Is there a library where this exists or a comparable way of doing similar?

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  • Any way to access the type of a Scala Option declaration at runtime using reflection?

    - by Graham Lea
    So, I have a Scala class that looks like this: class TestClass { var value: Option[Int] = None } and I'm tackling a problem where I have a String value and I want to coerce it into that Option[Int] at runtime using reflection. To do so, I need to know that the field is an Option and that the type parameter of the Option is Int. What are my options for figuring out that the type of 'value' is Option[Int] at runtime (i.e. using reflection)? I have seen similar problems solved by annotating the field, e.g. @OptionType(Int.class). I'd prefer a solution that didn't require annotations on the reflection target if possible.

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  • How do you write an idiomatic Scala Quicksort function?

    - by Don Mackenzie
    I recently answered a question with an attempt at writing a quicksort function in scala, I'd seen something like the code below written somewhere. def qsort(l: List[Int]): List[Int] = { l match { case Nil => Nil case pivot::tail => qsort(tail.filter(_ < pivot)) ::: pivot :: qsort(tail.filter(_ >= pivot)) } } My answer received some constructive criticism pointing out that List was a poor choice of collection for quicksort and secondly that the above wasn't tail recursive. I tried to re-write the above in a tail recursive manner but didn't have much luck. Is it possible to write a tail recursive quicksort? or, if not, how can it be done in a functional style? Also what can be done to maximise the efficiency of the implementation? Thanks in advance.

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  • How can I avoid mutable variables in Scala when using ZipInputStreams and ZipOutpuStreams?

    - by pr1001
    I'm trying to read a zip file, check that it has some required files, and then write all valid files out to another zip file. The basic introduction to java.util.zip has a lot of Java-isms and I'd love to make my code more Scala-native. Specifically, I'd like to avoid the use of vars. Here's what I have: val fos = new FileOutputStream("new.zip"); val zipOut = new ZipOutputStream(new BufferedOutputStream(fos)); while (zipIn.available == 1) { val entry = zipIn.getNextEntry if (entryIsValid(entry)) { val fos = new FileOutputStream("subdir/" + entry.getName()); val dest = new BufferedOutputStream(fos); // read data into the data Array var data: Array[Byte] = null var count = zip.read(data) while (count != -1) { dest.write(data, 0, count) count = zip.read(data) } dest.flush dest.close } }

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  • scala 2.8.0.RC2 compiler problem on pattern matching statement?

    - by gruenewa
    Why does the following module not compile on Scala 2.8.RC[1,2]? object Test { import util.matching.Regex._ val pVoid = """\s*void\s*""".r val pVoidPtr = """\s*(const\s+)?void\s*\*\s*""".r val pCharPtr = """\s*(const\s+)GLchar\s*\*\s*""".r val pIntPtr = """\s*(const\s+)?GLint\s*\*\s*""".r val pUintPtr = """\s*(const\s+)?GLuint\s*\*\s*""".r val pFloatPtr = """\s*(const\s+)?GLfloat\s*\*\s*""".r val pDoublePtr = """\s*(const\s+)?GLdouble\s*\*\s*""".r val pShortPtr = """\s*(const\s+)?GLshort\s*\*\s*""".r val pUshortPtr = """\s*(const\s+)?GLushort\s*\*\s*""".r val pInt64Ptr = """\s*(const\s+)?GLint64\s*\*\s*""".r val pUint64Ptr = """\s*(const\s+)?GLuint64\s*\*\s*""".r def mapType(t: String): String = t.trim match { case pVoid() => "Unit" case pVoidPtr() => "ByteBuffer" case pCharPtr() => "CharBuffer" case pIntPtr() | pUintPtr() => "IntBuffer" case pFloatPtr() => "FloatBuffer" case pShortPtr() | pUshortPtr() => "ShortBuffer" case pDoublePtr() => "DoubleBuffer" case pInt64Ptr() | pUint64Ptr() => "LongBuffer" case x => x } }

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  • How do I make lambda functions generic in Scala?

    - by Electric Coffee
    As most of you probably know you can define functions in 2 ways in scala, there's the 'def' method and the lambda method... making the 'def' kind generic is fairly straight forward def someFunc[T](a: T) { // insert body here what I'm having trouble with here is how to make the following generic: val someFunc = (a: Int) => // insert body here of course right now a is an integer, but what would I need to do to make it generic? val someFunc[T] = (a: T) => doesn't work, neither does val someFunc = [T](a: T) => Is it even possible to make them generic, or should I just stick to the 'def' variant?

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  • Scala loop returns as Unit and compiler points to "for" syntax?

    - by DeLongey
    Seems like Unit is the theme of my troubles today. I'm porting a JSON deserializer that uses Gson and when it comes to this for loop: def deserialize(json:JsonElement, typeOfT:Type, context:JsonDeserializationContext) = { var eventData = new EventData(null, null) var jsonObject = json.getAsJsonObject for(entry <- jsonObject.entrySet()) { var key = entry.getKey() var element = entry.getValue() element if("previous_attributes".equals(key)) { var previousAttributes = new scala.collection.mutable.HashMap[String, Object]() populateMapFromJSONObject(previousAttributes, element.getAsJsonObject()) eventData.setPreviousAttributes(previousAttributes) eventData } else if ("object".equals(key)) { val `type` = element.getAsJsonObject().get("object").getAsString() var cl = objectMap.get(`type`).asInstanceOf[StripeObject] var `object` = abstractObject.retrieve(cl, key) eventData.setObject(`object`) eventData } } } The compiler spits out the error type mismatch; found : Unit required: com.stripe.EventData and it points to this line here: for(entry <- jsonObject.entrySet()) Questions Confirm that it is indeed the Gson method entrySet() appearing as unit? If not, what part of the code is creating the issue? I've set return types/values for eventData class methods Is there a workaround for the Gson Unit issue? Thanks!

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  • Scala, represent pattern of boolean tuple into something else.

    - by Berlin Brown
    This is a cellular automata rule (input Boolean == Left, Center, Right Cell) and output Boolean . What is a better way to represent this in Scala. trait Rule { def ruleId() : Int def rule(inputState:(Boolean, Boolean, Boolean)) : Boolean override def toString : String = "Rule:" + ruleId } class Rule90 extends Rule { def ruleId() = 90 def rule(inputState:(Boolean, Boolean, Boolean)) : Boolean = { // Verbose version, show all 8 states inputState match { case (true, true, true) => false case (true, false, true) => false case (false, true, false) => false case (false, false, false) => false case _ => true } } }

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  • In scala can I pass repeated parameters to other methods?

    - by Fred Haslam
    Here is something I can do in java, take the results of a repeated parameter and pass it to another method: public void foo(String ... args){bar(args);} public void bar(String ... args){System.out.println("count="+args.length);} In scala it would look like this: def foo(args:String*) = bar(args) def bar(args:String*) = println("count="+args.length) But this won't compile, the bar signature expects a series of individual strings, and the args passed in is some non-string structure. For now I'm just passing around arrays. It would be very nice to use starred parameters. Is there some way to do it?

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  • How is `toString` in `scala.Enumeration$Value` implemented?

    - by Red Hyena
    I have an enum Fruit defined as: object Fruit extends Enumeration { val Apple, Banana, Cherry = Value } Now printing values of this enum, on Scala 2.7.x gives: scala> Fruit foreach println line1$object$$iw$$iw$Fruit(0) line1$object$$iw$$iw$Fruit(1) line1$object$$iw$$iw$Fruit(2) However the same operation on Scala 2.8 gives: scala> Fruit foreach println warning: there were deprecation warnings; re-run with -deprecation for details Apple Banana Cherry My question is: How is the method toString in Enumeration in Scala 2.8 is implemented? I tried looking into the source of Enumeration but couldn't understand anything.

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  • How can I pass a type as a parameter in scala?

    - by rsan
    I'm having a really hard time trying to figure out how can I store or pass a type in scala. What I want to achive is something like this: abstract class Foo( val theType : type ) object Foo{ case object Foo1 extends Foo(String) case object Foo2 extends Foo(Long) } So at some point I can do this: theFoo match{ case String => "Is a string" case Long => "Is a long" } and when obtaining the object being able to cast it: theFoo.asInstanceOf[Foo1.theType] Is this possible? If is possible, is a good aproach? What I'm trying to achieve ultimately is writing a pseudo schema for byte stream treatment. E.g if I have an schema Array(Foo1,Foo1,Foo2,Foo3,Foo1) I could parse Arrays of bytes that complain with that schema, if at some point I have a different stream of bytes I could just write a new schema Array(Foo3, Foo4, Foo5) without having to reimplement parsing logic. Regards,

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  • scala coalesces multiple function call parameters into a Tuple -- can this be disabled?

    - by landon9720
    This is a troublesome violation of type safety in my project, so I'm looking for a way to disable it. It seems that if a function takes an AnyRef (or a java.lang.Object), you can call the function with any combination of parameters, and Scala will coalesce the parameters into a Tuple object and invoke the function. In my case the function isn't expecting a Tuple, and fails at runtime. I would expect this situation to be caught at compile time. object WhyTuple { def main(args: Array[String]): Unit = { fooIt("foo", "bar") } def fooIt(o: AnyRef) { println(o.toString) } } Output: (foo,bar)

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  • How to yield a single element from for loop in scala?

    - by Julio Faerman
    Much like this question: Functional code for looping with early exit Say the code is def findFirst[T](objects: List[T]):T = { for (obj <- objects) { if (expensiveFunc(obj) != null) return /*???*/ Some(obj) } None } How to yield a single element from a for loop like this in scala? I do not want to use find, as proposed in the original question, i am curious about if and how it could be implemented using the for loop. * UPDATE * First, thanks for all the comments, but i guess i was not clear in the question. I am shooting for something like this: val seven = for { x <- 1 to 10 if x == 7 } return x And that does not compile. The two errors are: - return outside method definition - method main has return statement; needs result type I know find() would be better in this case, i am just learning and exploring the language. And in a more complex case with several iterators, i think finding with for can actually be usefull. Thanks commenters, i'll start a bounty to make up for the bad posing of the question :)

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  • Conflicting PACKAGE_NAME and other macros when using autotools.

    - by baol
    When using autotools (with a config.h file) for both a library and a software built on that library the compiler complains about a redefinition of some macros (PACKAGE_NAME, PACKAGE_TARNAME and so on). How can I prevent this? The config.h file is needed in the library to propagate it's setting to the software that use it. Right now I have a wrapper script library_config.h that includes the original config.h and provides defaults when the user is not using autotools, but even undefining the macros in that package I get the redefinition warning from gcc. #ifndef LIB_CONFIG_H #define LIB_CONFIG_H #ifdef HAVE_CONFIG_H # include "config.h" # undef PACKAGE # undef PACKAGE_BUGREPORT # undef PACKAGE_NAME # undef PACKAGE_STRING # undef PACKAGE_TARNAME # undef PACKAGE_VERSION # undef VERSION #else # if defined (WIN32) # define HAVE_UNORDERED_MAP 1 # define TR1_MIXED_NAMESPACE 1 # elif defined (__GXX_EXPERIMENTAL_CXX0X__) # define HAVE_UNORDERED_MAP 1 # else # define HAVE_TR1_UNORDERED_MAP 1 # endif #endif #endif I believe the best option would be to have a library without that macros: How can I avoid the definition of PACKAGE, PACKAGE_NAME and so on in the library when using autotools?

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  • Idiomatic Scala way to deal with base vs derived class field names?

    - by Gregor Scheidt
    Consider the following base and derived classes in Scala: abstract class Base( val x : String ) final class Derived( x : String ) extends Base( "Base's " + x ) { override def toString = x } Here, the identifier 'x' of the Derived class parameter overrides the field of the Base class, so invoking toString like this: println( new Derived( "string" ).toString ) returns the Derived value and gives the result "string". So a reference to the 'x' parameter prompts the compiler to automatically generate a field on Derived, which is served up in the call to toString. This is very convenient usually, but leads to a replication of the field (I'm now storing the field on both Base and Derived), which may be undesirable. To avoid this replication, I can rename the Derived class parameter from 'x' to something else, like '_x': abstract class Base( val x : String ) final class Derived( _x : String ) extends Base( "Base's " + _x ) { override def toString = x } Now a call to toString returns "Base's string", which is what I want. Unfortunately, the code now looks somewhat ugly, and using named parameters to initialize the class also becomes less elegant: new Derived( _x = "string" ) There is also a risk of forgetting to give the derived classes' initialization parameters different names and inadvertently referring to the wrong field (undesirable since the Base class might actually hold a different value). Is there a better way? Edit: To clarify, I really only want the Base values; the Derived ones just seem necessary for initializing the Base ones. The example only references them to illustrate the ensuing issues. It might be nice to have a way to suppress automatic field generation if the derived class would otherwise end up hiding a base class field.

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  • How do I define functions using PLT Scheme macros?

    - by nickname
    I am trying to write a macro that defines a special class of data structure with associated functions. I know this is possible; it is done multiple times in the core language itself. As a specific example, how would I define the define-struct macro in Scheme itself. It needs to create make-struct, struct-<<field>>, etc functions. I tried doing this using define, however, this only defines the function in the macro's lexical scope. How can I actually define a function in a macro?

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  • How do we simplify this kind of code in Java? Something like macros in C?

    - by Terry Li
    public static boolean diagonals(char[][] b, int row, int col, int l) { int counter = 1; // because we start from the current position char charAtPosition = b[row][col]; int numRows = b.length; int numCols = b[0].length; int topleft = 0; int topright = 0; int bottomleft = 0; int bottomright = 0; for (int i=row-1,j=col-1;i>=0 && j>=0;i--,j--) { if (b[i][j]==charAtPosition) { topleft++; } else { break; } } for (int i=row-1,j=col+1;i>=0 && j<=numCols;i--,j++) { if (b[i][j]==charAtPosition) { topright++; } else { break; } } for (int i=row+1,j=col-1;i<=numRows && j>=0;i++,j--) { if (b[i][j]==charAtPosition) { bottomleft++; } else { break; } } for (int i=row+1,j=col+1;i<=numRows && j<=numCols;i++,j++) { if (b[i][j]==charAtPosition) { bottomright++; } else { break; } } return topleft + bottomright + 1 >= l || topright + bottomleft + 1 >= l; //in this case l is 5 } After I was done posting the code above here, I couldn't help but wanted to simplify the code by merging the four pretty much the same loops into one method. Here's the kind of method I want to have: public int countSteps(char horizontal, char vertical) { } Two parameters horizontal and vertical can be either + or - to indicate the four directions to walk in. What I want to see if possible at all is i++; is generalized to i horizontal horizontal; when horizontal taking the value of +. What I don't want to see is if or switch statements, for example: public int countSteps(char horizontal, char vertical) { if (horizontal == '+' && vertical == '-') { for (int i=row-1,j=col+1;i>=0 && j<=numCols;i--,j++) { if (b[i][j]==charAtPosition) { topright++; } else { break; } } } else if (horizontal == '+' && vertical == '+') { for (int i=row+1,j=col+1;i>=0 && j<=numCols;i++,j++) { if (b[i][j]==charAtPosition) { topright++; } else { break; } } } else if () { } else { } } Since it is as tedious as the original one. Note also that the comparing signs for the loop condition i>=0 && j<=numCols; for example, >= && <= have correspondence with the value combination of horizontal and vertical. Sorry for my bad wording, please let me know if anything is not clear.

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  • How to flatten list of options using higher order functions?

    - by Synesso
    Using Scala 2.7.7: If I have a list of Options, I can flatten them using a for-comprehension: val listOfOptions = List(None, Some("hi"), None) listOfOptions: List[Option[java.lang.String]] = List(None, Some(hi), None) scala> for (opt <- listOfOptions; string <- opt) yield string res0: List[java.lang.String] = List(hi) I don't like this style, and would rather use a HOF. This attempt is too verbose to be acceptable: scala> listOfOptions.flatMap(opt => if (opt.isDefined) Some(opt.get) else None) res1: List[java.lang.String] = List(hi) Intuitively I would have expected the following to work, but it doesn't: scala> List.flatten(listOfOptions) <console>:6: error: type mismatch; found : List[Option[java.lang.String]] required: List[List[?]] List.flatten(listOfOptions) Even the following seems like it should work, but doesn't: scala> listOfOptions.flatMap(_: Option[String]) <console>:6: error: type mismatch; found : Option[String] required: (Option[java.lang.String]) => Iterable[?] listOfOptions.flatMap(_: Option[String]) ^ The best I can come up with is: scala> listOfOptions.flatMap(_.toList) res2: List[java.lang.String] = List(hi) ... but I would much rather not have to convert the option to a list. That seems clunky. Any advice?

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  • How can one make a 'passthru' function in C++ using macros or metaprogramming?

    - by Ryan
    So I have a series of global functions, say: foo_f1(int a, int b, char *c); foo_f2(int a); foo_f3(char *a); I want to make a C++ wrapper around these, something like: MyFoo::f1(int a, int b, char* c); MyFoo::f2(int a); MyFoo::f3(char* a); There's about 40 functions like this, 35 of them I just want to pass through to the global function, the other 5 I want to do something different with. Ideally the implementation of MyFoo.cpp would be something like: PASSTHRU( f1, (int a, int b, char *c) ); PASSTHRU( f2, (int a) ); MyFoo::f3(char *a) { //do my own thing here } But I'm having trouble figuring out an elegant way to make the above PASSTHRU macro. What I really need is something like the mythical X getArgs() below: MyFoo::f1(int a, int b, char *c) { X args = getArgs(); args++; //skip past implicit this.. ::f1(args); //pass args to global function } But short of dropping into assembly I can't find a good implementation of getArgs().

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  • How to further improve error messages in Scala parser-combinator based parsers?

    - by rse
    I've coded a parser based on Scala parser combinators: class SxmlParser extends RegexParsers with ImplicitConversions with PackratParsers { [...] lazy val document: PackratParser[AstNodeDocument] = ((procinst | element | comment | cdata | whitespace | text)*) ^^ { AstNodeDocument(_) } [...] } object SxmlParser { def parse(text: String): AstNodeDocument = { var ast = AstNodeDocument() val parser = new SxmlParser() val result = parser.parseAll(parser.document, new CharArrayReader(text.toArray)) result match { case parser.Success(x, _) => ast = x case parser.NoSuccess(err, next) => { tool.die("failed to parse SXML input " + "(line " + next.pos.line + ", column " + next.pos.column + "):\n" + err + "\n" + next.pos.longString) } } ast } } Usually the resulting parsing error messages are rather nice. But sometimes it becomes just sxml: ERROR: failed to parse SXML input (line 32, column 1): `"' expected but `' found ^ This happens if a quote characters is not closed and the parser reaches the EOT. What I would like to see here is (1) what production the parser was in when it expected the '"' (I've multiple ones) and (2) where in the input this production started parsing (which is an indicator where the opening quote is in the input). Does anybody know how I can improve the error messages and include more information about the actual internal parsing state when the error happens (perhaps something like a production rule stacktrace or whatever can be given reasonably here to better identify the error location). BTW, the above "line 32, column 1" is actually the EOT position and hence of no use here, of course.

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  • Macro to create macros?

    - by JMarsch
    Over the years, I've built up a number of macros that I like to have available in visual studio. It's always a pain to reload them and rebind them to the keyboard when I go to a different machine/rebuild/use a VM/etc. Someone mentioned to me once that there is a way that you can write a macro that will recreate your macros and bind them to keys automatically. Anyone know how to do that? Is there another way to easily export/import macros (nonsensically, VS has an "export macro" function, but no import).

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