Type Constructors

Type Constructors in Java and Scala

In "Functional Programming in Scala", I came across this code:

trait Foldable[F[_]] { ...

This is an example of something common in Java and Scala: type constructors.

"We write it as F[_], where the underscore indicates that F is not a type but a type constructor that takes one type argument. Just like functions that take other functions as arguments are called higher-order functions, something like Foldable is a higher-order type constructor or a higher-kinded type.

"Just like values and functions have types, types and type constructors have kinds. Scala uses kinds to track how many type arguments a type constructor takes, whether it’s co- or contravariant in those arguments, and what the kinds of those arguments are." [1]

However, each language differs in their use of type constructors.

"[I]n Scala it is not valid to say something is of type List, unlike in Java where javac would put the List<Object> for you. Scala is more strict here, and won’t allow us to use just a List in the place of a type, as it’s expecting a real type - not a type constructor." [2]

Java doesn't allow you to write something so abstract as the Foldable trait above. For instance, this won't compile:

public class Foldable<U<T>> { ...

Which brings us on to Types of a Higher Kind...

[1] Functional Programming in Scala

[2] Scala's Types of Types.