Cloning a tuple turns it into an array

[Zarel]

TypeScript Version: every version tested (from 2.3.0 to 2.5.0-dev.20170613)

Code

type Tuple = [number, number];
let t1: Tuple = [1, 2];
let t2: Tuple = t1.slice();
let t3: Tuple = t1.slice(0);
let t4: Tuple = [...t1];

Expected behavior:
No error

Actual behavior:

test-slice.ts(3,5): error TS2322: Type 'number[]' is not assignable to type '[number, number]'.
  Property '0' is missing in type 'number[]'.
test-slice.ts(4,5): error TS2322: Type 'number[]' is not assignable to type '[number, number]'.
test-slice.ts(5,5): error TS2322: Type 'number[]' is not assignable to type '[number, number]'.

array.slice() and array.slice(0) and [...array] are common idiomatic ways to shallow-clone an array. Unfortunately, this loses the tuple type information.

I don't know how easy this would be to fix, but even something like supporting only tuples below a certain length (by hardcoding [T1, T2] => [T1, T2] type signatures for Array#slice) would be a pretty big improvement for my own use-case.

[mhegazy]

I suppose we can make Tuple extend from a new Tuple interface instead of Array, and make that return this in a punch of places.

[thw0rted]

For some reason #4988 was closed but this was left open. I just want one correct, non-verbose way to clone a tuple.

[thw0rted]

I don't remember exactly what led me to post this a few months ago but as of now it looks like slice does return the same tuple. Unfortunately, so does slice(0,1) so you can easily give yourself a tuple that's actually shorter than should be guaranteed. Is that intentional?

[y-nk]

@thw0rted it seems not the case anymore at least in 4.8.0-dev

[thw0rted]

I don't remember the circumstances when I posted that, but it looks like I was probably wrong. I spent a minute trying different versions of the playground and all the ones I can find behave the same way -- slice returns an array.

[RyanCavanaugh]

Spread works now, which seems sufficient + idiomatic, and I think we don't have any good mechanisms to hardcode slice in a satisfying way.

[thw0rted]

Thanks Ryan, is this in the docs anywhere? I can't find anything about spread on the Tuple page, or anything about tuples on the Spread page.

I'm trying to understand how implied types work with spread, specifically. In this example, you can explicitly type an assignment from a spread-clone without any errors, but if you don't give any hints, it still winds up as a non-tuple array.

I'd still like to have a built-in that copies a tuple and always treats the result as the same tuple signature; I guess I could write a lightweight wrapper that just uses spread if I have to.

[RyanCavanaugh]

This is a facet of contextual typing: array literals get checked as tuples if contextually typed by a tuple type.