This is now supported (since ScalaTest 2.1.3) within interactive mode: testOnly *MySuite — -z foo to run only the tests whose name includes the substring “foo”. For exact match rather than substring, use -t instead of -z. If you run it from the command line, it should be as single argument to sbt: sbt ‘testOnly … Read more
Miles Sabin describes a very nice way to get union type in his recent blog post Unboxed union types in Scala via the Curry-Howard isomorphism: He first defines negation of types as type ¬[A] = A => Nothing using De Morgan’s law this allows him to define union types type ∨[T, U] = ¬[¬[T] with … Read more
I’m very interested in Scala, but I have not used it yet, so with that caveat, the frameworks I am aware of that are not mentioned in HRJ’s answer (Lift, Sweet, Slinky) are: Scalatra, previously Step (on GitHub) Play 2 (on GitHub) Pinky
In general, all 6 fold functions apply a binary operator to each element of a collection. The result of each step is passed on to the next step (as input to one of the binary operator’s two arguments). This way we can cumulate a result. reduceLeft and reduceRight cumulate a single result. foldLeft and foldRight … Read more
The preferred technique is to use pattern matching. This allows you to gracefully handle the case that the value in question is not of the given type: g match { case g2: Graphics2D => g2 case _ => throw new ClassCastException } This block replicates the semantics of the asInstanceOf[Graphics2D] method, but with greater flexibility. … Read more
EDIT: Java Conversions got @deprecated in Scala 2.13.0. Use scala.jdk.CollectionConverters instead. JavaConversions provide a series of implicit methods that convert between a Java collection and the closest corresponding Scala collection, and vice versa. This is done by creating wrappers that implement either the Scala interface and forward the calls to the underlying Java collection, or … Read more
The original answer discussing the code can be found below. First of all, you have to distinguish between different types of API, each with its own performance considerations. RDD API (pure Python structures with JVM based orchestration) This is the component which will be most affected by the performance of the Python code and the … Read more
These are called generalized type constraints. They allow you, from within a type-parameterized class or trait, to further constrain one of its type parameters. Here’s an example: case class Foo[A](a:A) { // ‘A’ can be substituted with any type // getStringLength can only be used if this is a Foo[String] def getStringLength(implicit evidence: A =:= … Read more
As a general rule, default to using Vector. It’s faster than List for almost everything and more memory-efficient for larger-than-trivial sized sequences. See this documentation of the relative performance of Vector compared to the other collections. There are some downsides to going with Vector. Specifically: Updates at the head are slower than List (though not … Read more
Few things to mention here, before giving the actual answer: Your question doesn’t have anything to do with left, it’s rather about the difference between reducing and folding The difference is not the implementation at all, just look at the signatures. The question doesn’t have anything to do with Scala in particular, it’s rather about … Read more