`collect` can be used to filter values on which the function is defined:

Get all values of type A:

    xs.collect { case a: A =&gt; a }

Get all values except B and C:

    xs diff xs.collect { case x@(_: B | _: C) =&gt; x }

`flatMap` that shit! ([as they say][1]):

    scala&gt; val ys = xs flatMap {
         |   case _: B | _: C =&gt; None
         |   case other =&gt; Some(other)
         | }
    ys: List[A] = List(D@7ecdc97b, E@2ce07e6b, E@468bb9d1)

In your case you were getting a `List[ScalaObject]` because `ScalaObject` is the least upper bound of `None`, `D`, and `E`.

  [1]: https://stackoverflow.com/q/8559537/334519

Formulating problems as questions seems to be a pretty good way of solving them :) My question actually provides the answer - just filter by subtype:


    val ys = xs filterNot(List(classOf[B], classOf[C]) contains _.getClass)


I&#39;m trying to do the following:

    Rename-Item c:\misc\*.xml *.tmp

I basically want to change the extension on every files within a directory to `.tmp` instead of `.xml`. I can&#39;t seem to find a straight forward way to do this in PowerShell.


How can I bulk rename files in PowerShell?

Here&#39;s my problem. I implemented a small script that does some heavy calculation, as a node.js module. So, if I type &quot;node myModule.js&quot;, it calculates for a second, then returns a value.
Now, I want to use that module from my main Node.JS program. I could just put all the calculation in a &quot;doSomeCalculation&quot; function then do:

    var myModule = require(&quot;./myModule&quot;);
    myModule.doSomeCalculation();

But that would be blocking, thus it&#39;d be bad. I&#39;d like to use it in a non-blocking way, like DB calls natively are, for instance. So I tried to use child_process.spawn and exec, like this:

    var spawn = require(&quot;child_process&quot;).spawn;
    var ext = spawn(&quot;node ./myModule.js&quot;, function(err, stdout, stderr) { /* whatevs */ });
    ext.on(&quot;exit&quot;, function() { console.log(&quot;calculation over!&quot;); });

But, of course, it doesn&#39;t work. I tried to use an EventEmitter in myModule, emitting &quot;calculationDone&quot; events and trying to add the associated listener on the &quot;ext&quot; variable in the example above. Still doesn&#39;t work.

As for forks, they&#39;re not really what I&#39;m trying to do. Forks would require putting the calculation-related code in the main program, forking, calculating in the child while the parent does whatever it does, and then how would I return the result?

So here&#39;s my question: can I use a child process to do some non-blocking calculation, when the calculation is put in a Node file, or is it just impossible? Should I do the heavy calculation in a Python script instead? In both cases, how can I pass arguments to the child process - for instance, an image?

How can I execute a node.js module as a child process of a node.js program?

I have a class structure like this

	abstract class A
	class B extends A
	class C extends A
	class D extends A
	class E extends A

and I have a collection of the various instances, for example:

	val xs = List(new D, new B, new E, new E, new C, new B)

My question is, is there an elegant way to filter out some of the subclasses from the List? 

Let&#39;s say I want all instances except B&#39;s and C&#39;s. I can do it with a bunch of isInstanceOf&#39;s, or using collect like this:

	val ys = (xs collect {
		case b: B =&gt; None
		case c: C =&gt; None
		case notBorC =&gt; notBorC
	}).filter(_ != None).asInstanceOf[List[A]]
                                   
This works but it feels awkward, mostly because of the filter and cast. Is there a more elegant way? Less code is preferred, and I&#39;d like to have a solution that doesn&#39;t need to be updated if I add more subclasses of A.

Filtering a Scala List by type

I have a class structure like this
<pre><code class="hljs language-java">abstract class A
class B extends A
class C extends A
class D extends A
class E extends A
</code></pre>
and I have a collection of the various instances, for example:
<pre><code class="hljs language-java">val xs = List(new D, new B, new E, new E, new C, new B)
</code></pre>
My question is, is there an elegant way to filter out some of the subclasses from the List?
Let's say I want all instances except B's and C's. I can do it with a bunch of isInstanceOf's, or using collect like this:
<pre><code class="hljs language-ini">val ys = (xs collect {
	case b: B => None
	case c: C => None
	case notBorC => notBorC
}).filter(_ != None).asInstanceOf[List[A]]
 
</code></pre>
This works but it feels awkward, mostly because of the filter and cast. Is there a more elegant way? Less code is preferred, and I'd like to have a solution that doesn't need to be updated if I add more subclasses of A.

I used to think that `private val` and `private final val` are same, until I saw section 4.1 in Scala Reference:


&gt; A constant value definition is of the form
&gt;
    final val x = e
&gt; where e is a constant expression (&#167;6.24). The final modifier must be present and no type annotation may be given. References to the constant value x are themselves treated as constant expressions; in the generated code they are replaced by the definition’s right-hand side e.

And I have written a test:

    class PrivateVal {
      private val privateVal = 0
      def testPrivateVal = privateVal
      private final val privateFinalVal = 1
      def testPrivateFinalVal = privateFinalVal
    }

`javap -c` output:

    Compiled from &quot;PrivateVal.scala&quot;
    public class PrivateVal {
      public int testPrivateVal();
        Code:
           0: aload_0       
           1: invokespecial #19                 // Method privateVal:()I
           4: ireturn       
    
      public int testPrivateFinalVal();
        Code:
           0: iconst_1      
           1: ireturn       
    
      public PrivateVal();
        Code:
           0: aload_0       
           1: invokespecial #24                 // Method java/lang/Object.&quot;&lt;init&gt;&quot;:()V
           4: aload_0       
           5: iconst_0      
           6: putfield      #14                 // Field privateVal:I
           9: return
    }

The byte code is just as Scala Reference said: `private val` is not `private final val`.

Why doesn&#39;t *scalac* just treat `private val` as `private final val`? Is there any underlying reason?


Why are `private val` and `private final val` different?

How do I &quot;join&quot; an iterable of strings by another string in Scala?

    val thestrings = Array(&quot;a&quot;,&quot;b&quot;,&quot;c&quot;)
    val joined = ???
    println(joined)

I want this code to output `a,b,c` (join the elements by &quot;,&quot;).

Scala: join an iterable of strings

Both types `Unit` and `Nothing` indicate a function that does not return anything. What&#39;s the difference between them?

What&#39;s the difference between Unit and Nothing?

In his talk [Simple Made Easy][1], Rick Hickey talks about &quot;**Polymorphism a la carte**&quot; (about 30:00 into the video). In the same context, he also mentions Haskell&#39;s [Type Classes][2] and Clojure&#39;s [Multi-Methods][3] (and protocols).

Since I am not very familiar with these concepts, I would like to understand its usefulness when trying to achieve **simplicity**. I am particularly interested in any examples or showcases of this concept in **Scala**. 


  [1]: http://www.infoq.com/presentations/Simple-Made-Easy
  [2]: http://en.wikipedia.org/wiki/Type_class
  [3]: http://en.wikipedia.org/wiki/Multiple_dispatch

What is &quot;polymorphism a la carte&quot; and how can I benefit from it?

I am trying to parse a CSV file, ideally using weka.core.converters.CSVLoader.
However the file I have is not a valid UTF-8 file.
It is mostly a UTF-8 file but some of the field values are in different encodings,
so there is no encoding in which the whole file is valid,
but I need to parse it anyway.
Apart from using java libraries like Weka, I am mainly working in Scala.
I am not even able to read the file usin scala.io.Source:
For example

    Source.
      fromFile(filename)(&quot;UTF-8&quot;).
      foreach(print);

throws:

        java.nio.charset.MalformedInputException: Input length = 1
	at java.nio.charset.CoderResult.throwException(CoderResult.java:277)
	at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:337)
	at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:176)
	at java.io.InputStreamReader.read(InputStreamReader.java:184)
	at java.io.BufferedReader.fill(BufferedReader.java:153)
	at java.io.BufferedReader.read(BufferedReader.java:174)
	at scala.io.BufferedSource$$anonfun$iter$1$$anonfun$apply$mcI$sp$1.apply$mcI$sp(BufferedSource.scala:38)
	at scala.io.Codec.wrap(Codec.scala:64)
	at scala.io.BufferedSource$$anonfun$iter$1.apply(BufferedSource.scala:38)
	at scala.io.BufferedSource$$anonfun$iter$1.apply(BufferedSource.scala:38)
	at scala.collection.Iterator$$anon$14.next(Iterator.scala:150)
	at scala.collection.Iterator$$anon$25.hasNext(Iterator.scala:562)
	at scala.collection.Iterator$$anon$19.hasNext(Iterator.scala:400)
	at scala.io.Source.hasNext(Source.scala:238)
	at scala.collection.Iterator$class.foreach(Iterator.scala:772)
	at scala.io.Source.foreach(Source.scala:181)

I am perfectly happy to throw all the invalid characters away or replace them with some dummy.
I am going to have lots of text like this to process in various ways
and may need to pass the data to various third party libraries.
An ideal solution would be some kind of global setting that would
cause all the low level java libraries to ignore invalid bytes in text,
so that that I can call third party libraries on this data without modification.

SOLUTION:

    import java.nio.charset.CodingErrorAction
    import scala.io.Codec

    implicit val codec = Codec(&quot;UTF-8&quot;)
    codec.onMalformedInput(CodingErrorAction.REPLACE)
    codec.onUnmappableCharacter(CodingErrorAction.REPLACE)

    val src = Source.
      fromFile(filename).
      foreach(print)


Thanks to +Esailija for pointing me in the right direction.
This lead me to https://stackoverflow.com/questions/3801890/how-to-detect-illegal-utf-8-byte-sequences-to-replace-them-in-java-inputstream
which provides the core java solution. In Scala I can make this the default behaviour by making the codec implicit. I think I can make it the default behaviour for the entire package by putting it the implicit codec definition in the package object.



Content Type	Original Author	Original Content on Stackoverflow
Question	hezamu	View Question on Stackoverflow
Solution 1 - Scala	Grozz	View Answer on Stackoverflow
Solution 2 - Scala	Travis Brown	View Answer on Stackoverflow
Solution 3 - Scala	hezamu	View Answer on Stackoverflow

Filtering a Scala List by type

Scala Problem Overview

Scala Solutions

Solution 1 - Scala

Solution 2 - Scala

Solution 3 - Scala

How can I execute a node.js module as a child process of a node.js program?

How can I bulk rename files in PowerShell?

Attributions