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  • Caching strategies for entities and collections

    - by Rob West
    We currently have an application framework in which we automatically cache both entities and collections of entities at the business layer (using .NET cache). So the method GetWidget(int id) checks the cache using a key GetWidget_Id_{0} before hitting the database, and the method GetWidgetsByStatusId(int statusId) checks the cache using GetWidgets_Collections_ByStatusId_{0}. If the objects are not in the cache they are retrieved from the database and added to the cache. This approach is obviously quick for read scenarios, and as a blanket approach is quick for us to implement, but requires large numbers of cache keys to be purged when CRUD operations are carried out on entities. Obviously as additional methods are added this impacts performance and the benefits of caching diminish. I'm interested in alternative approaches to handling caching of collections. I know that NHibernate caches a list of the identifiers in the collection rather than the actual entities. Is this an approach other people have tried - what are the pros and cons? In particular I am looking for options that optimise performance and can be implemented automatically through boilerplate generated code (we have our own code generation tool). I know some people will say that caching needs to be done by hand each time to meet the needs of the specific situation but I am looking for something that will get us most of the way automatically.

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  • Convert collections of enums to collection of strings and vice versa

    - by Michael Freidgeim
    Recently I needed to convert collections of  strings, that represent enum names, to collection of enums, and opposite,  to convert collections of   enums  to collection of  strings. I didn’t find standard LINQ extensions.However, in our big collection of helper extensions I found what I needed - just with different names: /// <summary> /// Safe conversion, ignore any unexpected strings/// Consider to name as Convert extension /// </summary> /// <typeparam name="EnumType"></typeparam> /// <param name="stringsList"></param> /// <returns></returns> public static List<EnumType> StringsListAsEnumList<EnumType>(this List<string> stringsList) where EnumType : struct, IComparable, IConvertible, IFormattable     { List<EnumType> enumsList = new List<EnumType>(); foreach (string sProvider in stringsList)     {     EnumType provider;     if (EnumHelper.TryParse<EnumType>(sProvider, out provider))     {     enumsList.Add(provider);     }     }     return enumsList;     }/// <summary> /// Convert each element of collection to string /// </summary> /// <typeparam name="T"></typeparam> /// <param name="objects"></param> /// <returns></returns> public static IEnumerable<string> ToStrings<T>(this IEnumerable<T> objects) {//from http://www.c-sharpcorner.com/Blogs/997/using-linq-to-convert-an-array-from-one-type-to-another.aspx return objects.Select(en => en.ToString()); }

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  • What type of webapp is the sweet spot for Scala's Lift framework?

    - by ajay
    What kind of applications are the sweet spot for Scala's lift web framework. My requirements: Ease of development and maintainability Ready for production purposes. i.e. good active online community, regular patches and updates for security and performance fixes etc. Framework should survive a few years. I don't want to write a app in a framework for which no updates/patches are available after 1 year. Has good UI templating engines Interoperation with Java (Scala satisfies this arleady. Just mentioning here for completeness sake) Good component oriented development. Time required to develop should be proportion to the complexity of web application. Should not be totally configuration based. I hate it when code gets automatically generated for me and does all sorts of magic under the hood. That is a debugging nightmare. Amount of Lift knowledge required to develop a webapp should be proportional to the complexity of the web application. i.e I should't have to spend 10+ hours learning Lift just to develop a simple TODO application. (I have knowledge of Databases, Scala) Does Lift satisfy these requirements?

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  • Why would a variable in Scala code mysteriously become null?

    - by Alex R
    I've isolated the problem down to this: Predef.println("the value of argv1 here is " + argv(1)); var n: $ = undef; n = argv(1); Predef.println("the value of argv1 here is " + argv(1)); Predef.println("the value of n here is " + n); Predef.println("the class of n here is " + n.getClass); Here's the definition of $: class $ { println("constructed a new $ of type: " + this.getClass); def value: $ = this; def toValue: Value = { new ConstStringValue(this.toString()) }; def -(sym: Symbol): $ = { println("looked up: " + sym); this } def -(sym: $): $ = { println("looked up: " + sym); this } def update(sym: Symbol, any: Any) { println("update called: " + sym + "=" + any); } def apply(sym: Symbol) = { this } def apply(obj: $) = { this } def apply() = { this } def +(o:$) = this.toValue.div(o.toValue) def *(o:$) = this.toValue.mul(o.toValue) def >(o:$) = this.toValue.gt(o.toValue) def <(o:$) = this.toValue.lt(o.toValue) def ++() = { this } def -=(o:$) = { this } } When run, the code prints: the value of argv1 here is 10 the value of argv1 here is 10 the value of n here is null java.lang.NullPointerException at test_1_php$.include(_tmp.scala:149) at php.script.main(php.scala:57) at test_1_php.main(_tmp.scala) [...] Why would n mysteriously lose its value (or fail to take one on)?

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  • Why does one of these statements compile in Scala but not the other?

    - by Jeff
    (Note: I'm using Scala 2.7.7 here, not 2.8). I'm doing something pretty simple -- creating a map based on the values in a simple, 2-column CSV file -- and I've completed it easily enough, but I'm perplexed at why my first attempt didn't compile. Here's the code: // Returns Iterator[String] private def getLines = Source.fromFile(csvFilePath).getLines // This doesn't compile: def mapping: Map[String,String] = { Map(getLines map { line: String => val pairArr = line.split(",") pairArr(0) -> pairArr(1).trim() }.toList:_*) } // This DOES compile def mapping: Map[String,String] = { def strPair(line: String): (String,String) = { val pairArr = line.split(",") pairArr(0) -> pairArr(1).trim() } Map(getLines.map( strPair(_) ).toList:_*) } The compiler error is CsvReader.scala:16: error: value toList is not a member of (St ring) = (java.lang.String, java.lang.String) [scalac] possible cause: maybe a semicolon is missing before `value toList'? [scalac] }.toList:_*) [scalac] ^ [scalac] one error found So what gives? They seem like they should be equivalent to me, apart from the explicit function definition (vs. anonymous in the nonworking example) and () vs. {}. If I replace the curly braces with parentheses in the nonworking example, the error is "';' expected, but 'val' found." But if I remove the local variable definition and split the string twice AND use parens instead of curly braces, it compiles. Can someone explain this difference to me, preferably with a link to Scala docs explaining the difference between parens and curly braces when used to surround method arguments?

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  • How Does One Make Scala Control Abstraction in Repeat Until?

    - by peter_pilgrim
    Hi I am Peter Pilgrim. I watched Martin Odersky create a control abstraction in Scala. However I can not yet seem to repeat it inside IntelliJ IDEA 9. Is it the IDE? package demo class Control { def repeatLoop ( body: = Unit ) = new Until( body ) class Until( body: = Unit ) { def until( cond: = Boolean ) { body; val value: Boolean = cond; println("value="+value) if ( value ) repeatLoop(body).until(cond) // if (cond) until(cond) } } def doTest2(): Unit = { var y: Int = 1 println("testing ... repeatUntil() control structure") repeatLoop { println("found y="+y) y = y + 1 } { until ( y < 10 ) } } } The error message reads: Information:Compilation completed with 1 error and 0 warnings Information:1 error Information:0 warnings C:\Users\Peter\IdeaProjects\HelloWord\src\demo\Control.scala Error:Error:line (57)error: Control.this.repeatLoop({ scala.this.Predef.println("found y=".+(y)); y = y.+(1) }) of type Control.this.Until does not take parameters repeatLoop { In the curried function the body can be thought to return an expression (the value of y+1) however the declaration body parameter of repeatUntil clearly says this can be ignored or not? What does the error mean?

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  • Is there a Scala version of .irbrc or another way to define some default libraries for REPL use?

    - by Tom Morris
    I've written a little library that uses implicits to add functionality that one only needs when using the REPL in Scala. Ruby has libraries like this - for things like pretty printing, firing up text editors (like the interactive_editor gem which invokes Vim from irb - see this post), debuggers and the like. The library I am trying to write adds some methods to java.lang.Class and java.lang.reflect classes using the 'pimp my library' implicit conversion process to help you go and find documentation (initially, with Google, then later possibly with a JavaDoc/ScalaDoc viewer, and maybe the StackOverflow API eventually!). It's an itch-scratching library: I spend so much time copying and pasting classnames into Google that I figured I may as well automate the process. It is the sort of functionality that developers will want to add to their system for use only in the REPL - they shouldn't really be adding it to projects (partly because it may not be something that their fellow developers want, but also because if you are doing some exploratory development, it may be with just a Scala REPL that's not being invoked by an IDE or build tool). In my case, I want to include a few classes and set up some implicits - include a .jar on the CLASSPATH and import it, basically. In Ruby, this is the sort of thing that you'd add to your .irbrc file. Other REPLs have similar ways of setting options and importing libraries. Is there a similar file or way of doing this for the Scala REPL?

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  • Why is my Scala function returning type Unit and not whatever is the last line?

    - by Andy
    I am trying to figure out the issue, and tried different styles that I have read on Scala, but none of them work. My code is: .... val str = "(and x y)"; def stringParse ( exp: String, pos: Int, expreshHolder: ArrayBuffer[String], follow: Int ) var b = pos; //position of where in the expression String I am currently in val temp = expreshHolder; //holder of expressions without parens var arrayCounter = follow; //just counts to make sure an empty spot in the array is there to put in the strings if(exp(b) == '(') { b = b + 1; while(exp(b) == ' '){b = b + 1} //point of this is to just skip any spaces between paren and start of expression type if(exp(b) == 'a') { temp(arrayCounter) = exp(b).toString; b = b+1; temp(arrayCounter)+exp(b).toString; b = b+1; temp(arrayCounter) + exp(b).toString; arrayCounter+=1} temp; } } val hold: ArrayBuffer[String] = stringParse(str, 0, new ArrayBuffer[String], 0); for(test <- hold) println(test); My error is: Driver.scala:35: error: type mismatch; found : Unit required: scala.collection.mutable.ArrayBuffer[String] ho = stringParse(str, 0, ho, 0); ^one error found When I add an equals sign after the arguments in the method declaration, like so: def stringParse ( exp: String, pos: Int, expreshHolder: ArrayBuffer[String], follow: Int ) ={....} It changes it to "Any". I am confused on how this works. Any ideas? Much appreciated.

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  • Why is Scala's type inferencer not able to resolve this?

    - by Levi Greenspan
    In the code snippet below - why do I have to give a type annotation for Nil? Welcome to Scala version 2.8.0.RC2 (OpenJDK Server VM, Java 1.6.0_18). Type in expressions to have them evaluated. Type :help for more information. scala> List(Some(1), Some(2), Some(3), None).foldLeft(Nil)((lst, o) => o match { case Some(i) => i::lst; case None => lst }) <console>:6: error: type mismatch; found : List[Int] required: object Nil List(Some(1), Some(2), Some(3), None).foldLeft(Nil)((lst, o) => o match { case Some(i) => i::lst; case None => lst }) ^ scala> List(Some(1), Some(2), Some(3), None).foldLeft(Nil:List[Int])((lst, o) => o match { case Some(i) => i::lst; case None => lst }) res1: List[Int] = List(3, 2, 1)

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  • Type classes or implicit parameters? What do you prefer and why? [closed]

    - by Petr Pudlák
    I was playing a bit with Scalaz and I realized that Haskell's type classes are very similar to Scala's implicit parameters. While Haskell passes the methods defined by a type class using hidden dictionaries, Scala allows a similar thing using implicit parameters. For example, in Haskell, one could write: incInside :: (Functor f) => f Int -> f Int incInside = fmap (+ 1) and the same function using Scalaz: import scalaz._; import Scalaz._; def incInside[F[_]](x: F[Int])(implicit fn: Functor[F]): F[Int] = fn.fmap(x, (_:Int) + 1); I wonder: If you could choose (i.e. your favorite language would offer both), what would you pick - implicits or type classes? And what are your pros/cons?

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  • Book (or resource) on Java bytecode

    - by Andrea
    I am looking for some resources on the JVM bytecode. Ideally I would for a short book; something more than a blog post but not a 800 pages tome. If it is relevant, I am a Scala developer, not a Java one, although I know Java just fine. I would like something that allowed me to read JVM bytecode and answer questions such as: Why does the bytecode has to know about high level construct such as classes? Are subtyping relations still visible in bytecode? How does type erasure work exactly? How do Oracle and Dalvik bytecode differ, and what consequences does this have for, say, developing Android apps with Scala? How does the JVM manage the stack, and why exactly this creates issues with tail call elimination? and so on.

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  • Constraints while designing the Java generics

    - by Andrea
    Java generics look quite different from those available in Scala, although both were designed by Martin Odersky. From my point of view, the design of generics in Java is worse, for instance: there is no possibility to specify variance one can get around the previous limitation by using wildcards, but this means the burden of specifying variance goes on the caller instead of the library designer one cannot use a type constructor in generics What were the constraints in Java that forced Odersky to design this mechanism for generics instead of the more flexible one he devised for Scala? Was he just savvier a few years later or there were actual limitations due to Java?

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  • Oracle ZS3 Contest for Partners: Share an unforgettable experience at the Teatro Alla Scala in Milan

    - by Claudia Caramelli-Oracle
    12.00 Dear valued Partner, We are pleased to launch a partner contest exclusive to our partners dedicated to promoting and selling Oracle Systems! You are essential to the success of Oracle and we want to recognize your contribution and effort in driving Oracle Storage to the market. To show our appreciation we are delighted to announce a contest, giving the winners the opportunity to attend a roundtable chaired by Senior Oracle Executives and spend an unforgettable evening at the magnificent Teatro Alla Scala in Milan, followed by a stay at the Grand Hotel et de Milan, courtesy of Oracle. Recognition will be given to 12 partner companies (10 VARs & 2 VADs) who will be recognized for their ZFS storage booking achievement in the broad market between June 1st and July 18th 2014. Criteria of Eligibility A minimum deal value of $30k is required for qualification Partners who are wholly or partially owned by a public sector organization are not eligible for participation  Winners The winning VARs will be: The highest ZS3 or ZBA bookings achievers by COB on July 18th, 2014 in each Oracle EMEA region (1) The highest Oracle on Oracle (2) ZS3 or ZBA bookings achievers by COB on July 18th, 2014 in each Oracle EMEA region The winning VADs (3) will be: The highest ZS3 or ZBA bookings achiever by COB on July 18th 2014 in EMEA The highest Oracle on Oracle (2) ZS3 or ZBA bookings achiever by COB on July 18th 2014 in EMEA  The Prize Winners will be invited to participate to a roundtable chaired by Oracle on Monday September 8th 2014 in Milan and to be guests of Oracle in the evening of September 8th, 2014 at the Teatro Alla Scala. The evening will comprise of a private tour of the Scala museum, cocktail reception at the elegant museum rooms and attending the performance by the renowned Soprano, Maria Agresta. Our guests will then retire for the evening to the Grand Hotel et de Milan, courtesy of Oracle. Oracle shall be the final arbiter in selecting the winners and all winners will be notified via their Oracle account manager.Full details about the contest, expenses covered by Oracle and timetable of events can be found on the Oracle EMEA Hardware (Servers & Storage) Partner Community workspace (FY15 Q1 ZFS Partner Contest). Remember: access to the community workspace requires membership. If you are not a member please register here. Good Luck!! For more information, please contact Sasan Moaveni. (1) Two VAR winners for each EMEA region – Eastern Europe & CIS, Middle East & Africa, South Europe, North Europe, UK/Ireland & Israel - as per the criteria outlined above (2) Oracle on Oracle, in this instance, means ZS3 or ZBA storage attached to DB or DB options, Engineered Systems or Sparc servers sold to the same customer by the same partner within the contest timelines.(3) Two VAD winners, one for each of the criteria outlined above, will be selected from across EMEA. Normal 0 14 false false false IT X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-fareast-language:EN-US;}

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  • Does C# have an equivalent to Scala's structural typing?

    - by Tom Morris
    In Scala, I can define structural types as follows: type Pressable { def press(): Unit } This means that I can define a function or method which takes as an argument something that is Pressable, like this: def foo(i: Pressable) { // etc. The object which I pass to this function must have defined for it a method called press() that matches the type signature defined in the type - takes no arguments, returns Unit (Scala's version of void). I can even use the structural type inline: def foo(i: { def press(): Unit }) { // etc. It basically allows the programmer to have all the benefits of duck typing while still having the benefit of compile-time type checking. Does C# have something similar? I've Googled but can't find anything, but I'm not familiar with C# in any depth. If there aren't, are there any plans to add this?

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  • Scala: Can I nudge a combinator parser to be locally greedy?

    - by eed3si9n
    Suppose I have an ambiguous language expressed in combinator parser. Is there a way to make certain expressions locally greedy? Here's an example of what I mean. import scala.util.parsing.combinator._ object Example extends JavaTokenParsers { def obj: Parser[Any] = (shortchain | longchain) ~ anyrep def longchain: Parser[Any] = zero~zero~one~one def shortchain: Parser[Any] = zero~zero def anyrep: Parser[Any] = rep(any) def any: Parser[Any] = zero | one def zero: Parser[Any] = "0" def one: Parser[Any] = "1" def main(args: Array[String]) { println(parseAll(obj, args(0) )) } } After compiling, I can run it as follows: $ scala Example 001111 [1.7] parsed: ((0~0)~List(1, 1, 1, 1)) I would like to somehow instruct the first part of obj to be locally greedy and match with longchain. If I switch the order around, it matches the longchain, but that's not because of the greediness. def obj: Parser[Any] = (longchain | shortchain) ~ anyrep

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  • When is a scala partial function not a partial function?

    - by Fred Haslam
    While creating a map of String to partial functions I ran into unexpected behavior. When I create a partial function as a map element it works fine. When I allocate to a val it invokes instead. Trying to invoke the check generates an error. Is this expected? Am I doing something dumb? Comment out the check() to see the invocation. I am using scala 2.7.7 def PartialFunctionProblem() = { def dream()() = { println("~Dream~"); new Exception().printStackTrace() } val map = scala.collection.mutable.HashMap[String,()=>Unit]() map("dream") = dream() // partial function map("dream")() // invokes as expected val check = dream() // unexpected invocation check() // error: check of type Unit does not take parameters }

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  • Scala and HttpClient: How do I resolve this error?

    - by Benjamin Metz
    I'm using scala with Apache HttpClient, and working through examples. I'm getting the following error: /Users/benjaminmetz/IdeaProjects/JakartaCapOne/src/JakExamp.scala Error:Error:line (16)error: overloaded method value execute with alternatives (org.apache.http.HttpHost,org.apache.http.HttpRequest)org.apache.http.HttpResponse <and> (org.apache.http.client.methods.HttpUriRequest,org.apache.http.protocol.HttpContext)org.apache.http.HttpResponse cannot be applied to (org.apache.http.client.methods.HttpGet,org.apache.http.client.ResponseHandler[String]) val responseBody = httpclient.execute(httpget, responseHandler) Here is the code with the error and line in question highlighted: import org.apache.http.client.ResponseHandler import org.apache.http.client.HttpClient import org.apache.http.client.methods.HttpGet import org.apache.http.impl.client.BasicResponseHandler import org.apache.http.impl.client.DefaultHttpClient object JakExamp { def main(args : Array[String]) : Unit = { val httpclient: HttpClient = new DefaultHttpClient val httpget: HttpGet = new HttpGet("www.google.com") println("executing request..." + httpget.getURI) val responseHandler: ResponseHandler[String] = new BasicResponseHandler val responseBody = httpclient.execute(httpget, responseHandler) // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ println(responseBody) client.getConnectionManager.shutdown } } I can successfully run the example in java...

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  • Can I create a collection in Scala that uses different equals/hashCode/compare implementations?

    - by Willis Blackburn
    I'm looking for as simple way to create an identity set. I just want to be able to keep track of whether or not I've "seen" a particular object while traversing a graph. I can't use a regular Set because Set uses "==" (the equals method in Scala) to compare elements. What I want is a Set that uses "eq." Is there any way to create a Set in Scala that uses some application-specified method for testing equality rather than calling equals on the set elements? I looked for some kind of "wrapEquals" method that I could override but did not find it. I know that I could use Java's IdentityHashMap, but I'm looking for something more general-purpose. Another idea I had was to just wrap each set element in another object that implements equals in terms of eq, but it's wasteful to generate tons of new objects just to get a new equals implementation. Thanks!

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  • Can i have a negative value as constant expression in Scala?

    - by Klinke
    I have an Java-Annotation that return a double value: @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD) public @interface DoubleValue { double value(); } When i try to attach the annotation to a field in a scala class and the value is negativ like here: class Test { @DoubleValue(-0.05) var a = _ } i get an compiler error with the message: "annotation argument needs to be a constant; found: 0.05.unary_-". I understood that i need a numerical literal and i looked into the Scala Language Specification and it seems, that the - sign is only used for the exponent but not for the mantissa. Does someone has an idea how i can have a negative value as runtime information using annotations? Thanks, Klinke

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  • How to make a Scala Applet whose Applet class is a singleton?

    - by Jamie
    Hi, I don't know if a solution exists but it would be highly desirable. I'm making a Scala Applet, and I want the main Applet class to be a singleton so it can be accessed elsewhere in the applet, sort of like: object App extends Applet { def init { // do init here } } Instead I have to make the App class a normal instantiatable class otherwise it complains because the contructor is private. So the ugly hack I have is to go: object A { var pp: App = null } class App extends Applet { A.pp = this def init { // do init here } } I really hate this, and is one of the reasons I don't like making applets in Scala right now. Any better solution? It would be nice...

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  • Overloading on return type ???

    - by Green Hyena
    scala> val shares = Map("Apple" -> 23, "MicroSoft" -> 50, "IBM" -> 17) shares: scala.collection.immutable.Map[java.lang.String,Int] = Map(Apple -> 23, MicroSoft -> 50, IBM -> 17) scala> val shareholders = shares map {_._1} shareholders: scala.collection.immutable.Iterable[java.lang.String] = List(Apple, MicroSoft, IBM) scala> val newShares = shares map {case(k, v) => (k, 1.5 * v)} newShares: scala.collection.immutable.Map[java.lang.String,Double] = Map(Apple -> 34.5, MicroSoft -> 75.0, IBM -> 25.5) From this example it seems like the map method is overloaded on return type. Overloading on return type is not possible right? Would somebody please explain what's going on here?

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