Questions tagged [scala-2.8]
Version 2.8 of the Scala language for the JVM. Use only if your question is specifically related to features of this version.
358
questions
872
votes
18
answers
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Is the Scala 2.8 collections library a case of "the longest suicide note in history"? [closed]
I have just started to look at the Scala collections library re-implementation which is coming in the imminent 2.8 release. Those familiar with the library from 2.7 will notice that the library, from ...
232
votes
4
answers
39k
views
Scala 2.8 breakOut
In Scala 2.8, there is an object in scala.collection.package.scala:
def breakOut[From, T, To](implicit b : CanBuildFrom[Nothing, T, To]) =
new CanBuildFrom[From, T, To] {
def apply(from: ...
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213
votes
4
answers
34k
views
What do <:<, <%<, and =:= mean in Scala 2.8, and where are they documented?
I can see in the API docs for Predef that they're subclasses of a generic function type (From) => To, but that's all it says. Um, what? Maybe there's documentation somewhere, but search engines don'...
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160
votes
19
answers
165k
views
How to write to a file in Scala?
For reading, there is the useful abstraction Source. How can I write lines to a text file?
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118
votes
4
answers
35k
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What is a "context bound" in Scala?
One of the new features of Scala 2.8 are context bounds. What is a context bound and where is it useful?
Of course I searched first (and found for example this) but I couldn't find any really clear ...
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113
votes
2
answers
82k
views
Overload constructor for Scala's Case Classes?
In Scala 2.8 is there a way to overload constructors of a case class?
If yes, please put a snippet to explain, if not, please explain why?
- 8,435
109
votes
6
answers
65k
views
How can I convert immutable.Map to mutable.Map in Scala?
How can I convert immutable.Map to mutable.Map in Scala so I can update the values in Map?
- 49.4k
97
votes
4
answers
39k
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Package objects
What are package objects, not so much the concept but their usage?
I've tried to get an example working and the only form I got to work was as follows:
package object investigations {
val ...
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86
votes
7
answers
25k
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What are Scala continuations and why use them?
I just finished Programming in Scala, and I've been looking into the changes between Scala 2.7 and 2.8. The one that seems to be the most important is the continuations plugin, but I don't understand ...
- 7,639
73
votes
1
answer
20k
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Scala 2.8 collections design tutorial
Following on from my breathless confusion, what are some good resources which explain how the new Scala 2.8 collections library has been structured. I'm interested to find some information on how the ...
- 134k
65
votes
1
answer
17k
views
Does the @inline annotation in Scala really help performance?
Or does it just clutter up the code for something the JIT would take care of automatically anyway?
- 11.6k
56
votes
6
answers
50k
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How to choose a random element from an array in Scala?
For example, there is a Scala array val A = Array("please", "help", "me"). How to choose a random element from this array?
- 2,103
52
votes
4
answers
45k
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What's the right way to use scala.io.Source?
In many examples, it is described that you can use scala.io.Source to read a whole file like this:
val str = scala.io.Source.fromFile("test.txt").mkString()
But closing the underlying stream is not ...
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52
votes
1
answer
15k
views
How do I exclude/rename some classes from import in Scala?
Language FAQ says
import scala.collection.mutable.{_, Map => _, Set => _}
should import all classes from package scala.collection.mutable, except Map and Set. But it gives me this error:
...
- 169k
51
votes
6
answers
31k
views
Scala Map implementation keeping entries in insertion order?
In Java, I use LinkedHashMap for this purpose. The documentation of Java's LinkedHashMap is very clear that it has "predictable iteration order" and I need the same in Scala.
Scala has ListMap and ...
- 7,817
50
votes
4
answers
57k
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How to convert a Seq[A] to a Map[Int, A] using a value of A as the key in the map?
I have a Seq containing objects of a class that looks like this:
class A (val key: Int, ...)
Now I want to convert this Seq to a Map, using the key value of each object as the key, and the object ...
- 205k
50
votes
1
answer
5k
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Function syntax puzzler in scalaz
Following watching Nick Partidge's presentation on deriving scalaz, I got to looking at this example, which is just awesome:
import scalaz._
import Scalaz._
def even(x: Int) : Validation[NonEmptyList[...
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49
votes
9
answers
34k
views
How to use Scala in IntelliJ IDEA (or: why is it so difficult to get a working IDE for Scala)?
I recently gave up trying to use Scala in Eclipse (basic stuff like completion doesn't work). So now I'm trying IntelliJ. I'm not getting very far.
I've been able to edit programs (within syntax ...
- 11.6k
40
votes
4
answers
34k
views
Scala foldLeft on Maps
How do you use Map.foldLeft? According to the docs it looks like
foldLeft [B] (z: B)(op: (B, (A, B)) ⇒ B) : B
But I'm having difficulty:
Map("first"->1,"second"->2).foldLeft(0)((a,(k,v)) =&...
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40
votes
17
answers
17k
views
Which IDE for Scala 2.8? [closed]
This is the same question for older version of Scala, but they say that Eclipse plugin has been improved vastly. Is it the best IDE now? How do different Scala IDE compare today?
- 49.4k
40
votes
4
answers
3k
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When is @uncheckedVariance needed in Scala, and why is it used in GenericTraversableTemplate?
@uncheckedVariance can be used to bridge the gap between Scala's declaration site variance annotations and Java's invariant generics.
scala> import java.util.Comparator
import java.util....
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36
votes
1
answer
21k
views
What's the new way to iterate over a Java Map in Scala 2.8.0?
How does scala.collection.JavaConversions supercede the answers given in Stack Overflow question Iterating over Java collections in Scala (it doesn't work because the "jcl" package is gone) and in ...
- 11.6k
31
votes
2
answers
6k
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Using scala.util.control.Exception
Does anybody have good examples of using scala.util.control.Exception version 2.12.0 (version 2.8.0), ? I am struggling to figure it out from the types.
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29
votes
4
answers
26k
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In Scala, is there a way to take convert two lists into a Map?
I have a two lists, a List[A] and a List[B]. What I want is a Map[A,B] but I want the semantics of zip. So started out like so:
var tuplesOfAB = listOfA zip listOfB
Now I'm not sure how to construct ...
- 1,873
28
votes
5
answers
17k
views
Convert Scala Set into Java (java.util.Set)?
I have a Set in Scala (I can choose any implementation as I am creating the Set. The Java library I am using is expecting a java.util.Set[String].
Is the following the correct way to do this in Scala ...
- 2,352
28
votes
2
answers
7k
views
Scala 2.8 CanBuildFrom
Following on from another question I asked, Scala 2.8 breakout, I wanted to understand a bit more about the Scala method TraversableLike[A].map whose signature is as follows:
def map[B, That](f: A =&...
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27
votes
7
answers
2k
views
Questions about Scala from a Rubyist
I have recently been looking around to learn a new language during my spare time and Scala seems to be very attractive.
I have a few questions regarding it:
Will not knowing Java impose a
challange ...
- 271
27
votes
2
answers
7k
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Implementing yield (yield return) using Scala continuations
How might one implement C# yield return using Scala continuations? I'd like to be able to write Scala Iterators in the same style. A stab is in the comments on this Scala news post, but it doesn't ...
- 16.2k
27
votes
3
answers
2k
views
What is the concept of "weak conformance" in Scala?
I just recently encountered the term "Weak Conformance" (in Stack Overflow user retronym's answer to How to set up implicit conversion to allow arithmetic between numeric types?).
What is it?
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26
votes
6
answers
7k
views
Writing functions of tuples conveniently in Scala
Quite a few functions on Map take a function on a key-value tuple as the argument. E.g. def foreach(f: ((A, B)) ⇒ Unit): Unit. So I looked for a short way to write an argument to foreach:
> val ...
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25
votes
3
answers
4k
views
What is the difference between a view and a stream?
In the Scala 2.8 collections framework, what is the difference between view and toStream?
- 26.6k
23
votes
3
answers
3k
views
What are nested/unnested packages in Scala 2.8?
In Scala 2.7, I could write:
package com.acme.bar
class Bar
.
package com.acme.foo
class Foo {
new bar.Bar
}
This doesn't compile in Scala 2.8 -- however this does:
package com.acme
package ...
- 54.9k
23
votes
5
answers
17k
views
Are there a good examples of using 'scala.swing'? [closed]
I don't know Java or Swing, but I'm quite familiar with Scala, and I have no problems using it for my purposes. But when I try to implement a GUI, I get a lot of problems/bugs that documentation can't ...
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23
votes
2
answers
1k
views
Don't understand the typing of Scala's delimited continuations (A @cpsParam[B,C])
I'm struggling to understand what precisely does it mean when a value has type A @cpsParam[B,C] and what types of this form should I assign to my values when using the delimited continuations facility....
22
votes
4
answers
18k
views
How to find a matching element in a list and map it in as an Scala API method?
Is there a method to do the following without doing both methods: find and map?
val l = 0 to 3
l.find(_ * 33 % 2 == 0).map(_ * 33) // returns Some(66)
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22
votes
3
answers
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What compromises Scala made to run on JVM?
Scala is a wonderful language, but I wonder how could be improved if it had it's own runtime?
I.e. what design choices were made because of JVM choice?
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22
votes
2
answers
13k
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Collection type generated by for with yield
When I evaluate a for in Scala, I get an immutable IndexedSeq (a collection with array-like performance characteristics, such as efficient random access):
scala> val s = for (i <- 0 to 9) yield ...
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21
votes
5
answers
2k
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What are the good Scala IDEs at the start of 2010?
I know this is an exact duplicate, but a year has gone by and Scala seems to be a fast moving thing, so I figure it might be acceptable to ask again:
What is the best IDE for Scala development right ...
- 79.6k
21
votes
4
answers
21k
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Scala: is it possible to override default case class constructor?
Just wondering if this is possible.
What I would actually like to do is check and possibly modify one of the arguments before it is stored as a val.
Alternatively, I could use an overload and make ...
- 2,867
20
votes
3
answers
4k
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Scala collection type for filter
Assume you have a List(1,"1") it is typed List[Any], which is of course correct and expected. Now if I map the list like this
scala> List(1, "1") map {
| case x: Int => x
| case y:...
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20
votes
3
answers
3k
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In Scala 2.8 collections, why was the Traversable type added above Iterable?
I know that to be Traversable, you need only have a foreach method. Iterable requires an iterator method.
Both the Scala 2.8 collections SID and the "Fighting Bitrot with Types" paper are basically ...
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19
votes
2
answers
16k
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Mixing Scala and Java files in an Eclipse project
I'm probably doing something stupid, but I can't spot it.
I've installed Eclipse Helios (Helios because I couldn't get Glassfish support to work correctly using Gallileo) and the nightly build of the ...
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18
votes
3
answers
10k
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Why does this explicit call of a Scala method allow it to be implicitly resolved?
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 ...
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18
votes
3
answers
3k
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Is it possible for an optional argument value to depend on another argument in Scala
Does anyone know if something like this is possible in Scala:
case class Thing(property:String)
def f(thing:Thing, prop:String = thing.property) = println(prop)
The above code doesn't compile; ...
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18
votes
3
answers
5k
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Proxies / delegates in Scala
I've seen several Scala questions recently (e.g. here, here, and here) that called for the use of proxies, and it's come up more than once in my own work. The Scala library has a number of proxy ...
- 21k
17
votes
2
answers
17k
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Scala Array constructor?
scala> val a = Array [Double] (10)
a: Array[Double] = Array(10.0)
scala> val a = new Array [Double] (10)
a: Array[Double] = Array(0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0)
Why these ...
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17
votes
1
answer
9k
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What is the most straightforward way to parse JSON in Scala?
I'm working on a simple web application with Scala. The plan is to obtain JSON data from an external API, and insert it into a template (unfortunately, obtaining the data in XML is not an option).
I'...
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17
votes
2
answers
8k
views
scala Remove (in place) all elements of a ListBuffer that meet a condition
I have a ListBuffer. I want to remove all elements that meet a certain condition.
I could iterate over it and remove each element. But what doe Scala say about mutating a list that you are iterating ...
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16
votes
2
answers
5k
views
Why does Iterator have a contains method but Iterable does not, in Scala 2.8?
I would like to call 'contains' on my Iterables :-)
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16
votes
2
answers
2k
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Scalaz: request for use case for Cokleisli composition
This question isn't meant as flame-bait! As it might be apparent, I've been looking at Scalaz recently. I'm trying to understand why I need some of the functionality that the library provides. Here's ...
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