Covariance

int[] iv = [1, 2, 3];

// Assigning `int[]` to `any[]` is allowed.
// The set of values allowed by `int` is a subset of set of values allowed by `any`
// The set of values allowed by `int[]` is a subset of set of values allowed by `any[]`
any[] av = iv;

public function main() {
    // A runtime error or else `iv[0]` would have the wrong type.
    av[0] = "str";

}

Covariance

Arrays and maps are covariant. Static type-checking guarantees that the result of a read from a mutable structure will be consistent with the static type. Covariance means that a write to a mutable structure may result in a runtime error. Arrays, maps, and records have an “inherent” type that constrains mutation.

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Type inference

int[] iv = [1, 2, 3];

any[] av = iv;

Assigning int[] to any[] is allowed. The set of values allowed by int is a subset of set of values allowed by any The set of values allowed by int[] is a subset of set of values allowed by any[]

public function main() {

    av[0] = "str";

A runtime error or else iv[0] would have the wrong type.

bal run covariance.bal
error: {ballerina/lang.array}InherentTypeViolation {"message":"incompatible types: expected 'int', found 'string'"}
        at covariance:main(covariance.bal:10)