What is a Singleton Type exactly?

by

If you think of a type as a set of values, the singleton type of a value x is the type which only contains this value ({x}). Usage examples:

  1. Pattern matching: case _: Foo.type checks that the matched object is the same as Foo using eq, where case Foo only checks that it’s equal to Foo using equals.

  2. It’s needed to write down the type of an object (as a type parameter, an argument, etc.):

    object A
def method(): A.type = A

  3. To guarantee the return value of a method is the object it’s called on (useful for method chaining, example from here):

    class A { def method1: this.type = { ...; this } }
class B extends A { def method2: this.type = { ...; this } }

    You can now call new B.method1.method2, which you couldn’t without this.type because method1 would return A.

More Related Contents:

  1. What is a TypeTag and how do I use it?
  2. Enforce type difference
  3. What is a higher kinded type in Scala?
  4. What are type lambdas in Scala and what are their benefits?
  5. Scala multiple type pattern matching
  6. Why is the Aux technique required for type-level computations?
  7. Are HLists nothing more than a convoluted way of writing tuples?
  8. Can’t prove that singleton types are singleton types while generating type class instance
  9. Turning A => M[B] into M[A => B]
  10. Static return type of Scala macros
  11. Scala type programming resources
  12. scala f-bounded types explanation
  13. Multiple parameter closure argument type not inferred
  14. What is the differences between Int and Integer in Scala?
  15. What is the eta expansion in Scala?
  16. How to define schema for custom type in Spark SQL?
  17. Specifying the filename when saving a DataFrame as a CSV [duplicate]
  18. Spark unionAll multiple dataframes
  19. Spark performance for Scala vs Python
  20. How to use regex to include/exclude some input files in sc.textFile?
  21. How to write spark streaming DF to Kafka topic
  22. sender inside a future
  23. When and why should one use Applicative Functors in Scala
  24. What’s the easiest way to use reify (get an AST of) an expression in Scala?
  25. Provide schema while reading csv file as a dataframe in Scala Spark
  26. Is there way to create tuple from list(without codegeneration)?
  27. What are Scala continuations and why use them?
  28. Error with varargs for function-objects in Scala?
  29. How to split a sequence into two pieces by predicate?
  30. Missing parameter type error by calling toSet?

Leave a Comment