[ReadOnlyArray] Array.isEmptyArray does not perform type narrowing

[samuelebonini]

Given a variable arr of type T[], I'm noticing that

import * as Array from "@effect/data/ReadonlyArray"

if(Array.isEmptyArray(arr)) {
    return;
}
arr; // <-- here arr still has type T[]

whereas

import * as Array from "@effect/data/ReadonlyArray"

if(!Array.isNonEmptyArray(arr)) {
    return;
}
arr; // <-- here arr has type [T, ...T[]]

Am I missing something? Shouldn't the first snippet also exhibit the same behavior, i.e. narrow the type down to a non-empty array?

[TylorS]

This is a TypeScript issue AFAIK. Refinement types are good at telling what a type is, but not good at what it isn't, unless the type you're refinining is a union. However, in this example, we're dealing subtypes of Arrays rather than unions.

EDIT: For example, we have similar kinds of code at my job:

[nandin-borjigin]

You can get the desired type safety at a cost of double negation (which I wouldn't trade for)

import { isNonEmptyArray } from '@effect/data/ReadonlyArray'


function foo<T>(arr: T[]) {
    if (!isNonEmptyArray(arr)) {
        return;
    }

    arr; // [T, ...T[]]
}

Or if you can live with nesting

import { isNonEmptyArray } from '@effect/data/ReadonlyArray'

function foo<T>(arr: T[]) {
    if (isNonEmptyArray(arr)) {
      arr; // [T, ...T[]]
    }

}

[tim-smart]

Unfortunately we can't fix this, as typescript only allows you to narrow based on what the type is, not what it isn't. Will close for now.