Scala maps -> operator

Question

What does the symbol -> mean in the context of a Map in Scala?

Scala’s Predef class offers an implicit conversion that lets one write key -> value as an alternate syntax for the pair (key, value). I read it in ScalaByExample but fail to see how this works for Maps.

Answer 1

Here's the implicit conversion:

implicit def any2ArrowAssoc[A](x: A): ArrowAssoc[A] = new ArrowAssoc(x) 

This will convert any type into an instance of "ArrowAssoc"

class ArrowAssoc[A](x: A) {     def -> [B](y: B): Tuple2[A, B] = Tuple2(x, y) } 

So when Scala sees

"a"->1 

It says "There is no method named '->' on String. Are there any implicit conversions in scope that can give me a type that has a method named '->'?" Predef.scala is automatically in scope, and offers a conversion to ArrowAssoc, which obviously has the '->' method. Scala then essentially converts the above to

any2ArrowAssoc("a").->(1) 

This method returns a Tuple2("a", 1) (often called a Pair). Map has a constructor that thats an array (varargs) of Tuple2s, so we're off to races! No magic in the compiler (besides implicit conversion, which is used extensively and for many different purposes), and no magic in Maps constructor.

Answer 2

The -> operator is a shorthand for associating a key with a value when used in the context of a map. In some languages, you are simply allowed to pass the pair of the key and value (usually with the key in the first position and the value in the second) to the constructor or one of the various methods on a map and it will be handled appropriately -- this is also allowed with Scala maps.

However, Scala also provides the shorthand form of key -> value, which, as you've discovered, is defined in Predef via an implicit, to make the association more clear. Essentially it's saying: "take the item on the left and map it to the item on the right." You'll notice if you read the definition of the Map object, that it does not define any methods that obviously use this -> method. Instead, the methods take objects of type Tuple2 (this is a tuple of 2 values: e.g. (item1, item2)), which Scala implicity converts via the method in Predef.

In some languages, the -> syntax is defined as part of the language itself, but due to the flexibility of Scala, this is able to be defined simply as a method.