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Type classes in Scala

Type classes are a powerful and flexible concept that adds ad-hoc polymorphism to Scala. They are not a first-class citizen in the language, but other built-in mechanisms allow writing them in Scala. This is the reason why they are not so obvious to spot in code and one can have some confusion over what the ‘correct’ way of writing them is.

This blog post summarizes the idea behind type classes, how they work and the way of coding them in Scala.

Idea

Type classes were first introduced in Haskell as a new approach to ad-hoc polymorphism. Philip Wadler and Stephen Blott describe it in How to make ad-hoc polymorphism less ad hoc. Type classes in Haskell are an extension to the Hindley–Milner type system, implemented by that language.

Type class is a class (group) of types, which satisfy some contract defined in a trait with addition that such functionality (trait and implementation) can be added without any changes to the original code. One could say that the same could be achieved by extending a simple trait, but with type classes it is not necessary to predict such a need beforehand.

There is no special syntax in Scala to express a type class, but the same functionality can be achieved using constructs that already exist in the language. That’s what makes it a little difficult for newcomers to spot a type class in code. A typical implementation of a type class uses some syntactic sugar as well, which also doesn’t make it clear right away what we are dealing with.

So let’s start with baby steps to implement a type class and understand it.

Implementation

Let’s write a type class that adds a function for getting the string representation of a given type. We add the ability for a given value to show itself. This is a .toString equivalent. We can start by defining a trait:

We want to have show functionality, but defined outside of each specific type definition. Let’s start by implementing show for an Int.

We have defined a companion object for Show to add functionality there. intCanShow holds an implementation of Show trait for Int. This is a just the first step. Of course, usage is still very cumbersome, to use this function we have to:

The full implementation containing all needed imports can be found in the repo.

The next step is to write the show function, in Show’s companion object, to avoid calling intCanShow explicitly.

The show function takes some parameter of type A and an implementation of the Show trait for that type A. Marking the intCanShow value as implicit makes the compiler able to find this implementation of Show[A] when there is a call to:

That is basically a type class. We’re going to massage it a little bit to make it look more like real code, but all the required parts are there. We have a trait that describes the functionality and implementations for each type we care about. There is also a function which applies an implicit instance’s function to the given parameter.

There is a more common way of writing the show function having an implicit parameter. Instead of writing:

we can use implicitly and rewrite it to:

We also used the context bound syntax: A: Show, which is a syntactic sugar in Scala, mainly introduced to support type classes, it basically does the rewrite we have done above (without the use of implicitly), more information can be found here.

There is one more trick (convention) often used in type classes. Instead of using implicitly we can add an apply function (to the Show companion object) with only an implicit parameter list:

and use it in show function:

This, of course, can be shortened even more:

We can improve our type class with the possibility of calling the show function as if it were a method on the given object - with a simple .show notation. By convention it is very often called a <TypeclassName>Ops class.

The Ops class allow us to write our client code like this:

To avoid a runtime overhead it is possible to make the ShowOps a value class and move the type class constraint to the show function, like this:

After some of the above rewrites, the companion object of Show looks like this:

Now we can add one more instance of our type class - for showing strings. It’s similar to the one showing ints of course.

In fact, this is so similar that we want to abstract it out - what could be done with a function to create instances for different types. We can rephrase it as a “constructor” for type class instances.

The above snippet presents a helper function instance that abstracts the common code and its usage for Int and String instances. With Scala 2.12 we can use Single Abstract Methods for this purpose instead, so the code is even more concise.

This is the simple type class that defines two ways of calling the show function (show() and .show). It also defines instances for two types: Int and String.

We may encounter a need to redefine some default type class instances. With the above implementation, when all default instances were imported into scope, we can not achieve that. The compiler will have ambiguous implicits in scope and will report an error.

We may decide to move the show function and the ShowOps implicit class to another object (let say ops) to allow users of this type class to redefine the default instance behavior (with Category 1 implicits, more on categories of implicits). After such a modification, the Show object looks like this:

Usage does not change, but now the user of this type class may import only:

Default implicit instances are not brought as Category 1 implicits (although they are available as Category 2 implicits), so it’s possible to define our own implicit instance where we use such a type class.

This is a basic type class that we have been coding from the very beginning.

Own types

Own types

The creator of a type class very often provides its instances for popular types, but our own types are not always supported out of the box (it depends on the library provider, whether some kind of products/coproducts derivation is implemented). Nothing stops us from writing our implementation for the type class. That, of course, looks exactly the same as if we would like to redefine, for some reason, the default instance that was provided by the implementer of the type class. The code implementing our own instance follows the same pattern, but could be implemented in a different location than the trait and the ops classes of the type class. On the other hand, if the type class is in our code base we may add this instance next to default instances defined in type class trait’s companion object. As an example, let’s define a way to show a Foo case class and its instance outside of the type class companion object:

Shapeless

This paragraph is a little off topic, but I think it’s worth mentioning.

The way type classes are implemented in Scala (with implicits) makes it possible to automatically derive type class instances for our own created types using Shapeless. For example, we could derive the show function (from previous paragraphs) for every case class (actually for every product type) defined in our code. We would need to define instances for basic types and how to define show for product types, but it will reduce so much boilerplate in our code!

Just for completeness of the information, similar derivation can be achieved with runtime reflection or compile-time macros.

Simulacrum

Simulacrum is a project that adds syntax for type classes using macros. Whether to use it or not depends on your personal judgement. If it is used, it’s trivial to find all type classes in our code and it reduces some boilerplate. On the other hand, a project that uses @typeclass has to depend on the macro paradise compiler plugin.

The equivalent of our Show example with an instance only for Int would look like this:

As you can see, the definition of a type class is very concise. On the usage side nothing changes - we would use it like this:

There is an additional annotation @op that may change the name of generated function and/or add some alias to generated method (i.e. |+| notation for summing).

Proper imports can be found in repo.

Implicits

Type classes use implicits as a mechanism for matching instances with code that uses them. Type classes come with all the benefits and costs related to implicits.

It is possible to define multiple instances of a type class for the same type. The compiler uses implicit resolution to find an instance that is the closest in the scope. As a comparison, a type class in Haskell can only have one instance. In Scala, we can define an instance and pass it as a parameter explicitly (not relying on implicit resolution), which makes the usage less convenient, but sometimes may be useful.

Our Show example needs a little modification to allow usage in a scenario, where we would like to pass instances explicitly. Let’s add a showExp function to the ShowOps class:

Now, it’s possible to just run the .showExp function or define and provide an instance of Show to showExp explicitly:

The first invocation uses the implicit found in scope, to the second invocation we pass the hipsterString instance of Show.

The other way (more common) to achieve the same result - without adding an extra function, but fully relying on implicits - is to create a Category 1 implicit that would take precedence over the default instance (a Category 2 implicit). This would look like this:

"baz" would use the default instance defined in Show, but "bazbaz" would use hipsterString instance.

The Scala way of implementing a type class (using implicits) could also cause some problems, which are described in the next paragraph.

Problems

With the power of implicits there comes a cost.

We can’t have two type class instances for some type T with the same precedence. This doesn’t sound like a terrible problem, but in practice it causes some real issues. It’s quite easy to get a compiler error (about ambiguous implicits) when using libraries like Cats or Scalaz, which heavily rely on type classes and build their types as a hierarchy (by subtyping). That is in detail described here. The problem is mainly related to the way type classes are implemented. Very often both ambiguous implicits implement exactly the same behavior, but the compiler can’t know about it. There are ongoing discussions on how to fix this.

Errors may also be misleading, because the compiler doesn’t know what a type class is, e.g. for our Show type class used in such a way:

compiler can only say that value show is not a member of Boolean.

A similar error message is even reported when ambiguous implicit definitions are found, but the .show notation was used.

Open source examples

Open source is a perfect place to look for examples of type classes. I would like to name two projects:

  • Cats

    Cats uses type classes as a primary way to model things. This lib uses also simulacrum. Instances are implemented in separate traits, also Ops are grouped in syntax traits.

  • Shapeless

    Shapeless heavily relies on type classes. The power of shapeless is the ability to work on HLists and derive type classes to add new functionality.

Future of type classes

There are different attempts and discussions on how to add syntax for type classes:

There are also some ongoing discussion on coherence of type classes:

Summary

Type classes as a concept are quite easy, but there are various corner cases when it comes to its implementation in Scala. The concept is rather used in libraries than in business applications, but it’s good to know type classes and potential risks of using them.

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