Incorrect typing when flip-flopping an array of tuples

[dsifford]

TypeScript Version: nightly (2.0.0-dev.201xxxxx)

Playground

Code

const x: [string,number][] = [
    ['one', 1],
    ['two', 2],
    ['three', 3]
];

// Type of y should be [number,string][]
// Instead, it is (number|string)[][]
const y = x.map(([a,b]) => [b,a]);

Expected behavior:

This may be an expected feature of the language, and if so, please disregard.

I expected the type of y to be [number,string][]

Actual behavior:

Type of y is (number|string)[][]

[weswigham]

As a workaround, you can cast your result:

const y = x.map(([a,b]) => ([b,a] as [number, string]));

Or, a personal favorite, write your own 'tuple constructor', which forces a tuple type to be inferred:

function tuple<A, B>(a: A, b: B): [A, B] {
    return [a, b];
}

const y = x.map(([a,b]) => tuple(b, a));

In any case, I think this is a duplicate of #6574, and discussion should probably continue there.

[dsifford]

Whoops! Didn't see the other discussion. Sorry about that.

And yep, I've been using casting. Totally not a giant issue for me, but just wanted to throw it out there.

Love the tuple constructor! Clever!

Thanks again 👍