Category Archives: Scala

How Scala implicits provide underscore.js / jQuery-style augmentations

Scala implicits allow one to write libraries like jQuery or underscore.js that add methods to existing classes without the syntactic overhead. This is very quick to demonstrate.

Consider the useful pluck method that underscore.js adds to Javascript arrays, used like so:

> _([{foo: 'baz'}, {foo: 'bizzle'}]).pluck("foo")
["baz", "bizzle"]

The underscore function sort of lets us “pretend” that pluck is a method of arrays of objects. Here is a similar method addition in Scala using implicits.

object Pluck {
  implicit def withPluck[T](ms: Seq[Map[String, T]]) = new {
    def	pluck(field: String) = ms.map(_.get(field))
  }
}

And here’s how you use it:

$ sbt console
scala> import Pluck._
scala> Seq(Map("foo" -> "baz"), Map("foo" -> "bizzle")) .pluck("foo")
res0: Seq[Option[java.lang.String]] = List(Some(baz), Some(bizzle))

In Scala, you can trivially “add” a method to an existing class without modifying that class or adding syntactic overhead. One may debate whether this is a good idea to do very often, of course, but clearly underscore.js is simply adding methods one really would expect an array to have.

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Heterogeneous Maps and Keys with Phantom Types in Scala

The term “heterogeneous collection” captures a few different ideas, most popularly a set or list where the elements may have different types. This post is not about those, but about a map (or dictionary, or key-value mapping, or whatever else you’d like to call it) where the values may be of different types, but the program does not lose that type information.

I’ve made a preliminary library from these code snippets at https://github.com/kennknowles/scala-heterogeneous-map. Let’s jump right in and do our imports…

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Type Classes in Scala

Type classes are an idea originally from Haskell that you can implement in Scala via implicit parameters. This is not new or obscure, but the technique is so useful that another post just demonstrating the basics can’t hurt. I will keep this brief — I have a lot to say about type classes, but first the fundamentals need to be extremely solid.

Let’s get imports out of the way….

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