scala-2.8 - w3toppers.com

When is @uncheckedVariance needed in Scala, and why is it used in GenericTraversableTemplate?

The problem is that GenericTraversableTemplate is used twice: once for mutable collections (where its type parameter should be invariant), and once for immutable collections (where covariance is invariably king). GenericTraversableTemplate’s typechecks assuming either covariance or invariance for the A type parameter. However, when we inherit it in a mutable trait, we have to pick invariance. … Read more

Which IDE for Scala 2.8? [closed]

I’ve been pretty successful with IDEA 9. I’ve briefly tried both Netbeans and Eclipse and wasn’t able to get what I wanted. Eclipse’s code-complete didn’t behave as well as I’d have liked, and I couldn’t find a way to make Netbeans handle Scala scripts; It’d just complain that the file wasn’t a class. To be … Read more

How can I convert immutable.Map to mutable.Map in Scala?

The cleanest way would be to use the mutable.Map varargs factory. Unlike the ++ approach, this uses the CanBuildFrom mechanism, and so has the potential to be more efficient if library code was written to take advantage of this: val m = collection.immutable.Map(1->”one”,2->”Two”) val n = collection.mutable.Map(m.toSeq: _*) This works because a Map can also … Read more

What are Scala continuations and why use them?

My blog does explain what reset and shift do, so you may want to read that again. Another good source, which I also point in my blog, is the Wikipedia entry on continuation passing style. That one is, by far, the most clear on the subject, though it does not use Scala syntax, and the … Read more

Overload constructor for Scala’s Case Classes?

Overloading constructors isn’t special for case classes: case class Foo(bar: Int, baz: Int) { def this(bar: Int) = this(bar, 0) } new Foo(1, 2) new Foo(1) However, you may like to also overload the apply method in the companion object, which is called when you omit new. object Foo { def apply(bar: Int) = new … Read more

Scala 2.8 CanBuildFrom

Note that the second argument to map is an implicit argument. There must be an implicit in scope with the appropriate types, or, otherwise, you must pass such an argument. In your example, That must be Set[String], B must be Int and Repr must be List[String]. Therefore, for that to compile you need the following … Read more

Implementing yield (yield return) using Scala continuations

Before we introduce continuations we need to build some infrastructure. Below is a trampoline that operates on Iteration objects. An iteration is a computation that can either Yield a new value or it can be Done. sealed trait Iteration[+R] case class Yield[+R](result: R, next: () => Iteration[R]) extends Iteration[R] case object Done extends Iteration[Nothing] def … Read more

Scala pattern matching with lowercase variable name

This is not specific to patterns with alternatives, and it is not a bug. An identifier that begins with a lowercase letter in a pattern represents a new variable that will be bound if the pattern matches. So, your example is equivalent to writing: x match { case MyValue1 | MyValue2 => println (“first match”) … Read more

Scala Map implementation keeping entries in insertion order?

From the LinkedHashMap Scaladoc page: “This class implements mutable maps using a hashtable. The iterator and all traversal methods of this class visit elements in the order they were inserted.”

Scala 2.8 collections design tutorial

Foreword There’s a 2.8 collection walk-through by Martin Odersky which should probably be your first reference. It has been supplemented as well with architectural notes, which will be of particular interest to those who want to design their own collections. The rest of this answer was written way before any such thing existed (in fact, … Read more