An intro to immutability with Dart

It’s impossible to deny that immutability is a hot topic in programming, especially front-end programming. Libraries like Immutable.js and other concepts like unidirectional data flow have argued it’s easier to reason about data when it doesn’t change underneath you:

In object-oriented and functional programming, an immutable object (unchangeable object) is an object whose state cannot be modified after it is created. This is in contrast to a mutable object (changeable object), which can be modified after it is created.

Well, what about Dart? We have a couple concepts that lend themselves well to immutability and immutable objects built-in, starting with the const modifier and const constructor. Not to be confused with const in ES6, which is merely an immutable binding:

In Dart, const is both an immutable binding and an immutable object:

All literals (Null, String, int, double, num, bool, Map, List, Symbol) are capable of being const, and it’s possible to create user-types with a const constructor:

Let’s review — const instances are both immutable bindings and enforced at a language level to be both deeply immutable — and also are canonicalized at compile-time — that is, any two instances are considered equivalent, and are only represented by a single instance when running. For example, the following is rather cheap — it only allocates a single instance at runtime:

Want to learn more? Read the Dart language tour about final and const.

Further static checking with package:meta

Of course, const is a bit restrictive — you must be able to create a class at compile-time — so you couldn’t for example read a database and create const objects at runtime. We’ve recently introduced the “immutable” annotation with package:meta:

You can use this annotation to help enforce that developers keep your classes deeply immutable. It won’t be canonicalized like const, but can still be quite helpful for developers.


I hope this was a nice introduction to immutability. Please let me know in the comments or on twitter if you’d like to learn more about Dart or immutability.