Fp-ts: Freeze empty from Array

[sutarmin]

🚀 Feature request

Hey! I want to suggest freezing empty to avoid occasional mutation (e.g. by third-parties).

Current Behavior

import { empty } from 'fp-ts/lib/Array';

function foo(arr: Array<number>) {
    arr.push(1); // might be not that obvious
}

foo(empty); // Neither compile-time nor runtime error
// All the code that depends on `empty` is broken after that

Desired Behavior

import { empty } from 'fp-ts/lib/Array';

function foo(arr: Array<number>) {
    arr.push(1);
}

foo(empty); // No compile-time error, but runtime exception

Suggested Solution

const empty: Array<never> = Object.freeze([]);

Who does this impact? Who is this for?

My team several times faced subtle bugs caused by empty been mutated occasionally by third-party code. I think that confidence that empty won't be mutated worth possible runtime exception at development time.

Describe alternatives you've considered

1) Adding readonly to empty's type - inconveniently restrict possible usages of empty cause you can't pass readonly Array<T> where you expect Array<T>
2) Object.seal - less strict than Object.freeze, but basically is the same for our case
3) Doing Object.freeze(empty) in every project - easy to forget about

Your environment

| Software | Version(s) |
| ---------- | ---------- |
| fp-ts | v2.1.0 |
| TypeScript | v3.6.3 |

[raveclassic]

2 cents here - if we disable mutations on Array then return type of empty should be ReadonlyArray. Otherwise it's a broken contract.

[raveclassic]

related https://github.com/gcanti/fp-ts/pull/544