Flix supports scoped mutable memory. In Flix, all mutable memory belongs to a region that is tied to its lexical scope. When execution leaves the lexical scope of a region, all memory in that region becomes unreachable.

Regions are useful because they enable us to implement pure functions that internally uses mutation. We will illustrate this powerful idea with several real-world examples, but let us first discuss how to use a region:

We introduce a new region scope with the region construct:

region rc { // region starts.
  ...       // the region handle `rc` is in scope.
}           // region ends and all data associated with `rc` is no longer in scope.

We can use regions to implement a pure sort function that internally uses mutation:

def sort(l: List[a]): List[a] with Order[a] =
    region rc {
        let arr = List.toArray(rc, l);

Here we introduce a region named rc. We use the function Array.toArray to convert the list l to a mutable array arr associated with the region rc. We then sort arr using Array.sort! which uses an efficient in-place sorting algorithm. Finally, we convert the sorted array back to a list and return it. The sort function is pure, even though it internally uses mutation.

As another example, we can implement a toString function for List[a] which is pure but internally uses a mutable StringBuilder:

def toString(l: List[a]): String with ToString[a] =
    region rc {
        let sb =;
        List.forEach(x -> StringBuilder.appendString!("${x} :: ", sb), l);
        StringBuilder.appendString!("Nil", sb);
    } // scope of rc ends, the entire expression is pure.

The programming pattern is the same: We open a new region, allocate a StringBuilder in the region, fill the builder with strings, and convert it into one string.

We can use regions to implement certain functional operations more efficiently. For example, here is a fast implementation of List.flatMap:

def flatMap(f: a -> List[b] \ ef, l: List[a]): List[b] \ ef =
    region rc {
        let ml =;
        l |> List.forEach(x -> MutList.append!(f(x), ml));

Regions are Values

A region (or region handle) is a value that can be passed as a function argument. This is useful, for example, when we want to write a reusable function that allocates and returns a mutable data structure.

For example, here is the List.toMutDeque function:

def toMutDeque(rc: Region[r], l: List[a]): MutDeque[a, rc] \ rc =
    let d =;
    forEach(x -> MutDeque.pushBack(x, d), l);

The function takes a region handle rc, allocates a new mutable deque (MutDeq) in the given region, inserts all elements of the list l in the deque, and returns it.

Regions are Scoped

Regions and all memory associated with them cannot outlive their lexical scope.

Consider the following program:

def main(): Unit \ IO = 
    let escaped = region rc {
        Array#{1, 2, 3} @ rc

Here we allocate the Array#{1, 2, 3} in the region rc and try to return it outside of its enclosing scope. The Flix compiler detects such escape violations and reports an error:

❌ -- Type Error ----------------------------

>> The region variable 'rc' escapes its scope.

2 |>     let escaped = region rc {
3 |>         Array#{1, 2, 3} @ rc
4 |>     };

region variable escapes.