While Flix recommends the use of immutable data structures (such as immutable lists, sets, and maps), mutable arrays may be useful for performance critical code.

We recommend that arrays are used sparingly and that when possible their use is hidden as an implementation detail. For example, the Flix Datalog engine uses arrays internally but exposes a functional (immutable) interface.

Flix uses monomorphization and consequently primitive arrays are not boxed. For example, the representation of an Array[Int32] is compact and efficient.

All operations on arrays are impure. As such, all functions that use arrays must be marked as impure (with annotation \ IO) or be casted to pure. However, accessing the length of an array is pure since the size of an array cannot change after it has been created.

Arrays should only be used for low-level code. The MutList data structure, available in the standard library, provides a mutable dynamically-expanding data structure similar to java.util.ArrayList. Its implementation is backed by an array that is dynamically resized and it provides amortized O(1) push operations.

Array Literals

An array literal is of the form [e1, e2, ... en]. For example, the expression:

[1, 2, 3, 4]

evaluates to an array with the four elements: 1, 2, 3, 4.

In some cases it is useful to allocate a large array filled with the same value. The expression:

["Hello World"; 100]

evaluates to an array of length 100 where every entry contains the string "Hello World".

Design Note

Flix does not allow the allocation of an array without assigning a "default value" to each entry in the array.

Reading and Writing from Arrays

Arrays can be accessed and updated using standard syntax. For example:

let a = [0; 10];
a[0] = 21;
a[1] = 42;
a[0] + a[1]

evaluates to 63, as expected.

Array Slicing

Arrays can be sliced. Slicing an array (shallowly) copies a subrange of the array. For example:

let a = [1, 2, 3, 4, 5];

evaluates to the array [3, 4].

The start or end index may be omitted. For example:

let a = [1, 2, 3, 4, 5];
let a1 = a[2..]; // evaluates to [3, 4, 5]
let a2 = a[..4]  // evaluates to [1, 2, 3, 4]

If both the start and end index are omitted the entire array is copied. For example:

let a = [1, 2, 3, 4, 5];

evaluates to the (copied) array [1, 2, 3, 4, 5].

Design Note

Slicing an array using the same start and end index returns the empty array. For example, [0, 1, 2, 3][2..2] evaluates to [].


Slicing with negative indices is undefined and results in runtime errors.

Array Length

The length of an array is accessed as follows:

let a = [1, 2, 3, 4, 5];

which evaluates to 5.