Scala idiom: Prefer immutable code (immutable data structures)

(I originally wrote this blog post in 2012, and it seems like it has held up well over time.)

One great thing I’ve learned from Scala and functional programming over the last few months is this: 

Make your variables immutable, unless there’s a good reason not to.

“Immutable” means that you can’t change (mutate) your variables; you mark them as final in Java, or use the val keyword in Scala. More important than you not changing your variables is that other programmers can’t change your variables, and you can’t change theirs.

If I write a Java method like this:

public void doSomething(Foo foo) {
  // do something here

I can modify the variable foo, even if I have no reason to. It’s an accident waiting to happen. Writing immutable code in Java means I should write that function like this, explicitly telling the caller of my function I won’t mess with their foo:

public void doSomething(final Foo foo) {
  // do something here

The problem with Java is that you have to add the final keyword to your method signature to make fields immutable, and because that requires more typing and makes your code more verbose, most Java developers don't bother to add it where a variable is meant to be immutable.

Immutable code in Scala

In Scala, the opposite approach is taken; with functions, function arguments are val by default, so no extra typing is needed.

For example, if I create a function that takes an Int and returns the value of that Int plus one, I can write it like this:

scala> def addOne(i: Int): Int = i + 1 
foo: (i: Int)Int

In that function, the variable i that is passed in is assumed to be a val. I can confirm that by trying to modify my function, like this:

scala> def addOne(i: Int): Int = i += 1; i
<console>:7: error: reassignment to val
         def addOne(i: Int): Int = i += 1; i

This brings me back to the point of this discussion, the Scala idiom of preferring immutable code (immutable data structures):

Prefer val over var. Only use var when you have to.

By writing your code this way, you remove the chance for subtle bugs to creep into the system. If you think of a function signature as being a contract, in Scala var means a function may mutate a variable, and val means it won’t.

It’s more than just val versus var

While I’m trying to keep this blog post short, I need to say that this discussion is about much more than just val versus var. Scala takes this much further, with a collections hierarchy that splits the collections into “immutable” and “mutable” data structures. 

Beyond that, the Scala Predef object automatically puts the immutable data structures (such as Map and Set) into your default environment, so if you create a new Map or Set, you automatically get the immutable structures. Just like using the final keyword in Java, if you want a mutable Map or Set, you’re going to have to type more and manually import it.

Scala immutability - quotes from the experts

I’ll leave you with a couple of quotes from Scala books. First, from Beginning Scala, by David Pollack:

“Using vals in your code makes you think about alternative, immutable, functional code. This small example demonstrates that removing vars leads to refactoring. The refactoring leads to new coding patterns. New coding patterns leads to a shift in your approach to programming. This shift in approach leads to transformative code that has fewer defects and is easier to maintain.”

From the excellent book, Programming in Scala, by Odersky, Spoon, and Venners, there is a small section titled, “A balanced attitude for Scala programmers”:

“Prefer vals, immutable objects, and methods without side effects. Reach for them first. Use vars, mutable objects, and methods with side effects when you have a specific need and justification for them.”

Scala idiom: Prefer immutable code - Summary

In summary, I hope this discussion of focusing on the Scala and functional programming idiom of focusing on writing immutable code has been helpful. I have only been working with Scala for a few months myself, and already I have seen a reduction in bugs from following this approach, and I highly recommend it.