The concept is wrong (or I don't get it)

There is an example in the readme:

const numbers: Array<number> = [];

// HegelError: Type "Array<number>" is incompatible with type "Array<number | string>"
const numbersOrStrings: Array<string | number> = numbers;

The error is completely wrong and here is why. The type Array<number> means «an array which every element is a number». The type Array<string | number> means «an array which every element is a string or a number». The first type is assignable to the second because «a number» is «a string or a number». For example, array [1, 2, 3] matches the description «an array which every element is a string or a number». Thus numbers CAN be assigned to numbersOrStrings.

The second part of the example is correct. The example gives only 1 error in runtime (the one in the second part).

This lack of flexibility is useless and bring inconveniences.

[Finesse]

@thecotne This is pretty clear, thank you.

Just in case, there is a syntax in TypeScript for immutable arrays:

function countSum(arr: readonly <Array<string | number>>) {

It means that a value can’t be add/changed/removed from the array through square brackets or the Array prototype methods. This syntax looks more pleasant than $Immutable<>.

[Finesse]

Sounds reasonable, my bad.