More on Sealed Traits in Scala

Published on 04 June 2015

A version of this post appeared on the Underscore blog.

My recent post on sealed traits had some great feedback but it was clear that I glossed over some points too quickly. In this post I want to go over a new trick I’ve learned for sealed traits and clear up some of the points I made in the last post.

The Product with Serializable Trick

If you declare algebraic data types with the pattern I gave in the last post you might see the compiler inferring types that unexpectedly include Product and Serializable. For example if you declare

sealed trait Color
final case object Red extends Color
final case object Green extends Color
final case object Blue extends Color

when you put elements into a list the compiler will infer the elements have type Product with Serializable with Color.

scala> val colors = List(Red, Green, Blue)
colors: List[Product with Serializable with Color] = List(Red, Green, Blue)

This happens because all case classes automatically extend Product and Serializable. To avoid this ugly type we can make our base type (the sealed trait) extend these two types as well.

sealed trait Color extends Product with Serializable
final case object Red extends Color
final case object Green extends Color
final case object Blue extends Color

Now we don’t see Product and Serializable in the inferred type.

val colors = List(Red, Green, Blue)
colors: List[Color] = List(Red, Green, Blue)

Thanks to Julian Truffaut and Channing Walton for pointing this out to me.

Why You Want to Use Final

I suggested you should mark case classes in algebraic data types as final, but didn’t explain very clearly why.

A final case class cannot be extended by any other class. This means you can make stronger guarantees about how your code behaves. You know that nobody can subclass your class, override some methods, and make something goofy happen. This is great when you are debugging code – you don’t have to go hunting all over the object hierarchy to work out which methods are actually being called.

Of course making classes final does mean that you lose a form of extensibility. If you do find yourself wanting to allow users to implement functionality you should wrap that functionality up in a trait and use the type class pattern instead.

Base Types vs Sub-types

In the previous post I rather cryptically said

Finally, when declaring types we almost always use the base type (e.g. `Option`) instead of a subtype (e.g. `None`) so the lack of exhaustiveness checking is very rarely an issue.

Say you define an algebraic data type

sealed trait Base
final case class Foo(a: Int) extends Base
final case class Bar(a: String) extends Base

You should almost always just refer to the Base trait when you declare types, not the subtypes. Some good reasons:

  • to ensure you get exhaustiveness checking;
  • to avoid issues with invariant type classes and invariant containers like Future; and
  • it’s often just doesn’t make sense to declare a subtype (why would a method ever declare return type as None?).

As I explained in the previous article, only sealed types turn on exhaustiveness checking. In the example over this means that values with type Base will have exhaustiveness checking but values of type Foo and Bar will not. That’s one good reason to declare method, return, or variable types as Base.

Type classes are one of the other big patterns I advocate for Scala. For reasons I won’t go into here (look out for a future blog post) invariant type classes generally work best in Scala. In an invariant type class means that TypeClass[Foo] is not a subtype of TypeClass[Base] and this will cause annoyance. Just use Base everywhere and the problem goes away.

You might have already run into this issue if you use invariant containers such as Future.

Finally, there often isn’t a lot of sense in declaring something to have a type that is not the base type of an algebraic data type. For example, if a method were to only return None it should just return (). If it was to return only Some it should just return the value wrapped in the Some.

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