Hidden features of Scala

Question

What are the hidden features of Scala that every Scala developer should be aware of?

One hidden feature per answer, please.

Answer 1

require method (defined in Predef) that allow you to define additional function constraints that would be checked during run-time. Imagine that you developing yet another twitter client and you need to limit tweet length up to 140 symbols. Moreover you can't post empty tweet.

def post(tweet: String) = {
  require(tweet.length < 140 && tweet.length > 0) 
  println(tweet)
 }

Now calling post with inappropriate length argument will cause an exception:

scala> post("that's ok")
that's ok

scala> post("")
java.lang.IllegalArgumentException: requirement failed
    at scala.Predef$.require(Predef.scala:145)
    at .post(<console>:8)

scala> post("way to looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong tweet") 
java.lang.IllegalArgumentException: requirement failed
    at scala.Predef$.require(Predef.scala:145)
    at .post(<console>:8)

You can write multiple requirements or even add description to each:

def post(tweet: String) = {
  require(tweet.length > 0, "too short message")
  require(tweet.length < 140, "too long message")
  println(tweet)
}

Now exceptions are verbose:

scala> post("")
java.lang.IllegalArgumentException: requirement failed: too short message
    at scala.Predef$.require(Predef.scala:157)
    at .post(<console>:8)

One more example is here.

---

Bonus

You can perform an action every time requirement fails:

scala> var errorcount = 0
errorcount: Int = 0

def post(tweet: String) = {
  require(tweet.length > 0, {errorcount+=1})
  println(tweet)
  }

scala> errorcount
res14: Int = 0

scala> post("")
java.lang.IllegalArgumentException: requirement failed: ()
    at scala.Predef$.require(Predef.scala:157)
    at .post(<console>:9)
...

scala> errorcount
res16: Int = 1

Answer 2

Dunno if this is really hidden, but I find it quite nice.

Typeconstructors that take 2 type parameters can be written in infix notation

object Main {                                                                   
  class FooBar[A, B]

  def main(args: Array[String]): Unit = {
    var x: FooBar[Int, BigInt] = null
    var y: Int FooBar BigInt   = null
  }
}

Answer 3

placeholder syntax for anonymous functions

From The Scala Language Specification:

SimpleExpr1 ::= '_'

An expression (of syntactic category Expr) may contain embedded underscore symbols _ at places where identifiers are legal. Such an expression represents an anonymous function where subsequent occurrences of underscores denote successive parameters.

From Scala Language Changes:

_ + 1                  x => x + 1
_ * _                  (x1, x2) => x1 * x2
(_: Int) * 2           (x: Int) => x * 2
if (_) x else y        z => if (z) x else y
_.map(f)               x => x.map(f)
_.map(_ + 1)           x => x.map(y => y + 1)

Using this you could do something like:

def filesEnding(query: String) =
  filesMatching(_.endsWith(query))

Answer 4

Build infinite data structures with Scala's Streams : http://www.codecommit.com/blog/scala/infinite-lists-for-the-finitely-patient