Scala Type Bounds

Working with types in Scala can be challenging in many ways. The deeper you dig into the complexity of this subject, the more difficult is to find the right path. In this short post, I will be explaining some of the most fundamental of Scala type system — the type bounds. Enjoy!

What is type bound?

A type bound is nothing more than a mechanism to add restrictions to the type parameters. When passing a type to let’s say a class, we can limit to what types we want to allow. This mechanism allows for greater control over our code and hopefully making it more robust.

There are 3 distinct type bounds available in Scala — upper bound, lower bound and upper and lower combined. The chapters below explain how to use each of them.

Upper type bound

An upper type bound constraints a type A to be a subtype of B. The Scala notation looks like this:

class SomeClass[A <: B]

The example above defines the type of A as a subtype of B. To understand it better, let's use some better example:

trait Eatable
class Vegetable extends Eatable
class Meat extends Eatable
class Potato extends Vegetable
class Beef extends Meat
class Orange extends Eatable
class Vegan[A <: Vegetable]
class Carnivore[A <: Meat]
class Food[A <: Eatable]
val vegan = new Vegan[Potato] // works fine
val mixed = new Carnivore[Potato] // error - type argument doesn't conform to type parameter bound
val all = new Food[Beef] // all good

As seen on a few examples above, the mechanism is pretty easy to use. The class Vegan, requires its type parameter to a subtype of Vegetable, which is exactly what Potato is. On the other hand, class Carnivore requires its parameter type to be a subtype of Meat, which Potato isn't hence the error. The Food class accepts any class that extends Eatable trait - a Beef is one of them.

Lower type bound

A lower type bounds works as one would expect — it limits the type A to be a supertype of B. The notation looks like this:

class OtherClass[A >:  B]

The notation is similar to previous bound — only the less than sign is replaced with greater than sign. Let’s use the classes from previous examples to see the lower type bound in practice:

trait Eatable
class Vegetable extends Eatable
class Meat extends Eatable
class Potato extends Vegetable
class Beef extends Meat
class Orange extends Eatable
class Stew[A >: Potato]
class BBQ[A >: Beef]
class Juice[A >: Orange]
val stew = new Stew[Vegetable] // works fine
val BBQ = new BBQ[Meat] // fine as well
val juice = new Juice[Potato] // error

The lower type bound is similarly easy to use. We specify three more concrete classes: Stew, BBQ, and Juice. Stew requires its type parameter to a supertype of Potato, for BBQ the type parameter should be a supertype of Beef and for Juice - supertype of Orange. First to instantiations work fine, but the last one fails - the Potato is by no means a supertype of Orange, therefore an error is thrown.

Lower and upper type bound

Last example is actually a combination of previous two. Scala allows to define both lower and upper bounds in the same time. The notation for this looks like this:

class LastClass[A <: B >: C]

In this example, the type A, has to be a subtype of B, while also being a supertype of C. As complicated as it may sound, it will be easier to understand using our 'food' examples:

trait Eatable
class Vegetable extends Eatable
class Meat extends Eatable
class Potato extends Vegetable
class Beef extends Meat
class Orange extends Eatable
class MarisPiper extends Potato
class Wagyu extends Beef
class Curacao extends Orange
class RedMeat[A <: Meat >: Wagyu]
class RootVegetable[A <: Vegetable >: MarisPiper]
val stew = new RedMeat[Beef] // all good
val BBQ = new RootVegetable[Potato] // works fine
val juice = new RootVegetable[MarisPiper] // error

This example should be fairly self-explanatory — we have two classes that use both upper and lower type bounds for their type parameters: RedMeat and RootVegetable. From the classes defined, the Beef works for the first one, and Potato for the second one. The Meat doesn't meet the upper type bound and Wagyu the lower type bound for RedMeat class type parameters.

Summary

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