NonEmptyArray<A> is not assignable to [A, ...A[]]

[geoffreytools]

🐛 Bug report

Current Behavior

NonEmptyArray<A> is not assignable to [A, ...A[]].

Expected behavior

One should be able to use NonEmptyArray (and ReadonlyNonEmptyArray) with functions using the open-ended tuple syntax for their type constraints.

Reproducible example

import { NonEmptyArray, of } from 'fp-ts/lib/NonEmptyArray';

const foo = (arr: [unknown, ...unknown[]]) => arr;

foo(of(0));
//  ~~~~~
// Argument of type 'NonEmptyArray<number>' is not assignable
// to parameter of type '[unknown, ...unknown[]]'

playground

Suggested solution(s)

- export interface NonEmptyArray<A> extends Array<A> {
-    0: A
- }
+ export type NonEmptyArray<A> = [A, ...A[]];

Your environment

Which versions of fp-ts are affected by this issue? Did this work in previous versions of fp-ts?

Software	Version(s)
fp-ts	I don't know. NonEmptyArray2v was introduced in 1.15.0
TypeScript	at least since 3.3.3

[orangebokov]

it wouldn't be bad to see this in fp-ts