Mutable Data

Flix is a functional-first programming language that encourages but does not demand, the use of immutable data structures. While immutable data structures should be the default, Flix has rich support for imperative programming with destructive updates to mutable data.

Flix uses its effect system to separate pure and impure code. In particular, Flix uses the region concept to track the use of mutable memory. That is, all mutable memory belongs to some statically-scoped region.

Flix has three basic types of mutable memory:

• References
• Arrays
• Structs

We can use these data types to build higher-level mutable data structures. For example, the Flix Standard Library offers collections such as MutList, MutDeque, MutSet, and MutMap. As a rule, these higher-level data structures should be preferred over lower-level references and arrays.

We begin this chapter with a discussion of regions.