val-mutable versus var-immutable in Scala

by

Pretty common question, this one. The hard thing is finding the duplicates.

You should strive for referential transparency. What that means is that, if I have an expression “e”, I could make a val x = e, and replace e with x. This is the property that mutability break. Whenever you need to make a design decision, maximize for referential transparency.

As a practical matter, a method-local var is the safest var that exists, since it doesn’t escape the method. If the method is short, even better. If it isn’t, try to reduce it by extracting other methods.

On the other hand, a mutable collection has the potential to escape, even if it doesn’t. When changing code, you might then want to pass it to other methods, or return it. That’s the kind of thing that breaks referential transparency.

On an object (a field), pretty much the same thing happens, but with more dire consequences. Either way the object will have state and, therefore, break referential transparency. But having a mutable collection means even the object itself might lose control of who’s changing it.

More Related Contents:

  1. What is Scala’s yield?
  2. How do I apply the enrich-my-library pattern to Scala collections?
  3. What is the difference between Scala’s case class and class?
  4. What is the motivation for Scala assignment evaluating to Unit rather than the value assigned?
  5. Difference between reduce and foldLeft/fold in functional programming (particularly Scala and Scala APIs)?
  6. Abort early in a fold
  7. What is the difference between def foo = {} and def foo() = {} in Scala?
  8. Scala currying vs partially applied functions
  9. Instantiating immutable paired objects
  10. Using Either to process failures in Scala code
  11. When to use parenthesis in Scala infix notation
  12. When and why should one use Applicative Functors in Scala
  13. Function returns an empty List in Spark
  14. Functional programming, Scala map and fold left [closed]
  15. What is a function literal in Scala?
  16. How to sort a list in Scala by two fields?
  17. Can I zip more than two lists together in Scala?
  18. Cross product in Scala
  19. Functional Reactive Programming in Scala [closed]
  20. Using partial functions in Scala – how does it work?
  21. Scala – initialization order of vals
  22. How to store custom objects in Dataset?
  23. What are type lambdas in Scala and what are their benefits?
  24. How do I break out of a loop in Scala?
  25. How to convert A[B[C]] to B[A[C]] if A and B are monads?
  26. Why are `private val` and `private final val` different?
  27. Accessing value returned by scala futures
  28. ScalaTest in sbt: is there a way to run a single test without tags?
  29. How to vectorize DataFrame columns for ML algorithms?
  30. Getting subclasses of a sealed trait

Leave a Comment