Unison Data Types: Explained

If you're a programmer, you've likely heard of Unison, the innovative functional programming language developed by Paul Chiusano and Rúnar Bjarnason. Unison is unique in several ways, but perhaps one of the most interesting is its approach to data types. In this article, we'll explore the world of Unison data types and their benefits.

What are Unison data types?

Unison is a data-first programming language, meaning that all data in the language is typed. In other words, Unison distinguishes between different types of data (e.g., numbers, strings, booleans) and enforces those distinctions. This is in contrast to dynamically-typed languages like Python, where the type of a variable can change at runtime.

Unison has a wide range of built-in data types, including primitive types like Int (integer), Float, Boolean, and Text. It also has more complex types such as List, Set, Map, Tuple, and Record.

How do Unison data types work?

Unison types are defined using algebraic data types (ADTs), a familiar concept for functional programmers. An ADT is a collection of data types and functions that operate on those data types. In Unison, ADTs are defined using a combination of constructor functions and pattern matching.

For example, let's say we want to define a data type for representing the suits in a deck of cards. We could define it like this:

type Suit = Hearts | Diamonds | Clubs | Spades

This defines a type Suit with four possible values: Hearts, Diamonds, Clubs, or Spades. The | symbol is used to separate the possible values, which are called constructors.

To use this type, we could create a variable mySuit and assign it a value like this:

let mySuit = Hearts

We could also define functions that operate on Suit values, like this one that returns the color of a given suit:

let colorOf(s: Suit) = 
  match s with 
  | Hearts => Red
  | Diamonds => Red
  | Clubs => Black
  | Spades => Black

Here, the match keyword is used to pattern match on the possible values of the Suit type. Depending on the value of s, the function will return either Red or Black.

This is just a small taste of what's possible with Unison data types. The language allows for much more complex types and patterns, making it an incredibly flexible and expressive language.

Why are Unison data types important?

Unison's data-first approach is part of what makes it such a powerful programming language. By enforcing type distinctions at compile time, Unison helps prevent bugs and improve reliability. It also makes code easier to reason about, as the types of all values are explicit and well-defined.

In addition, Unison's ADT-based approach to types allows for more expressive and flexible data structures. ADTs allow for complex types that can be composed and combined in a variety of ways. This makes it easier to create data structures that are tailored to specific needs, rather than relying on general-purpose structures like arrays or dictionaries.

Finally, Unison's approach to types makes it easier to work with data across different environments and platforms. Because all data is typed, it can be serialized and deserialized without losing information. This makes data exchange between different systems much easier, which is especially important in modern, distributed systems.


Unison data types are an important part of what makes Unison such an innovative and powerful programming language. By enforcing type distinctions at compile time and allowing for complex ADTs, Unison helps prevent bugs, improve reliability, and create more flexible data structures. If you're interested in functional programming or the power of typed languages, Unison is definitely worth exploring.

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Written by AI researcher, Haskell Ruska, PhD (haskellr@mit.edu). Scientific Journal of AI 2023, Peer Reviewed