Scala – tagged types

Data types in a programming language are the description or classification of the data that instructs the compiler how to treat the data. Of course, they are not only for the compiler or interpreter but also for us, the developers, as they help us understand the code big time.

This is a valid definition of the data which type is Map[String, String]:

This is the valid definition for our domain because both, booking and payment ids have the type String. Also for this trivial example, the type definition looks perfectly fine and pretty enough. However, we can imagine that in a more complex situation, in a bigger codebase we may lack some information when we see definitions like this:

As a result, it is not that rare that we see comments like this:

We also quickly notice that it is not only about the readability of our code but also about the safety. This code perfectly compiles but it is not valid in our domain, it introduces a well-hidden bug:

What if we would like to add some extra information to the type definition? Something like “metadata” for the types? The information that not only helps the developers to comprehend the code but also introduces additional “type safety” by the compiler/interpreter.

The solution is there in the Scala ecosystem and it is called Tagged types.

There are two main implementations of Tagged typesScalaz and Shapeless but also SoftwareMill’s Scala Common implementation should be mentioned. In this post, I will shortly show the usage of Shapeless Tagged types.

This is how the simple model that we convert to the new one using tagged types looks like:

The final code should look close to this:

We can already see that this code is easier to comprehend:

but what we do not see yet is the fact that we also introduced the additional “type safety”.

How to get to that second implementation? Let’s do it step by step.

First, we need to create the tags:

These are simple Scala traits but actually other types can be used here as well. However, the trait is most convenient. The names have suffix Tag by convention.

We can use those tags to create tagged types:

This is basically how we tag the types. We say what type (String, Int, etc.) is tagged by which tag (String @@ StringTag, Int @@ IdTag, etc.).

But with this code we are still a bit far from our desired implementation. It is clear that these parts are boilerplate:

We can easily replace them with type aliases (also presented in the previous post):

With this implementation, we are very close to what we would expect but this code still does not compile:

This says that we are using String type in the parameter which is expected to be a tagged type:

The constructor (apply() method) of Booking case class expects tagged type but we supplied it with simple String. To fix this we need to make sure that we create the instance of the tagged type. This is how it can be done:

This is how we defined instances of tagged types:

Now the code compiles.

The code is also on the github.


Let summarize what we achieved here:

  • the intention of the code is clearly visible:

We know immediately that the bookingPaymentMapping maps booking ids to payment ids.

  • we get errors in the compilation time when we accidentally switch the ids:


The examples presented in this post are trivial but even though we see the clear benefits of using tagged types. Imagine the complex project and I think we are fully convinced that this is really usefuly technique for every Scala developer toolset.

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3 years ago

Hi, Nice.
I’m doing something almost identical but I use also something like

package object tags {
type BookingId = String @@ BookingIdTag
object BookingId {
def apply(s: String): BookingId = tag[BookingIdTag][String](s)

which allows the rather more succinct
val bookingId1: BookingId = BookingId("bookingId1")

3 years ago

It seems that tagged types is more heavyweight alternative to ‘newtype’-s in Haskell. Am I right?

3 years ago
Reply to  Nikolay

It’s not very heavyweight. The tagging is compile time only so at runtime they revert to the untagged raw types. So an Int is still an Int. For example, there’s no runtime space overhead for arrays of tagged ints – it’s still the unboxed primitive

3 years ago
Reply to  Nikolay

Thanks for explanation, guys! Btw, I found library that generalizes type tagging approach to defining newtypes in Scala:

3 years ago

Thanks for detailed reply!

3 years ago

Why not just use value classes here?

3 years ago
Reply to  Damian

Hi, nice article!
It’s not clear what is the overhead associated with value classes since official documentation says the following: “The type at compile time is Wrapper, but at runtime, the representation is an Int”.
Moreover in Correctness section, it says: “Another use case for value classes is to get the type safety of a data type without the runtime allocation overhead”.

So what’s the catch?

Alexander Semenov
Alexander Semenov
3 years ago
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