Typescript: Add a way to prefer narrowing array to tuple if possible

[Jamesernator]

Search Terms

tuple narrow, prefer tuple

Suggestion

This is a meta-suggestion over the top of some of the cases in #16896 and #26113, basically at current it's quite repetitive to specify overloads for tuple types and we tend to wind up things like this:

// etc up to how ever many overloads might be needed
function zipLongest<T1, T2, T3, T4>(
  iterables: [Iterable<T1>, Iterable<T2>, Iterable<T3>, Iterable<T4>]
): Iterable<[T1, T2, T3, T4]>

function zipLongest([]): []
function zipLongest<T>(iterables: Array<Iterable<T>>): Iterable<Array<T>>

We can already specify the type if we know only tuples are going to be input e.g.:

type Unwrap<T> = T extends Iterable<infer R> ? R : never
type ZipUnwrapped<T> = { [P in keyof T]: Unwrap<T[P]> }

declare function zipLongest<Ts extends Array<Iterable<any>>>(iterables: Ts): Iterable<ZipUnwrapped<Ts>>

const strs = ['foo', 'bar', 'baz', 'boz']
const nums = [1, 2, 3, 4]
const zipPair: [string[], number[]] = [strs, nums]
const z = zipLongest(zipPair)

In this case the type of z is correctly inferred to be Iterable<[string, number]>, but if we remove the type declaration on zipPair then we get that the inferred type of z is Iterable<(number | string)[]>.

Basically what I propose is adding some way to specify that an array should be narrowed to the most precise tuple possible if we can.

There's a couple approaches that might be used:

Approach 1

Use the [...TupleParam] spread syntax already mentioned in #26113 and when this syntax is seen narrow to the best tuple we could get. So the above declaration would become:

// The only difference is iterables: Ts has become iterables: [...Ts]
declare function zipLongest<Ts extends Array<Iterable<any>>>(iterables: Ts): Iterable<ZipUnwrapped<Ts>>

This has the downside that it might for any Ts in [Some, Tuple, Params, ...Ts] to be narrowed even when the wider type was what was intended.

Approach 2

Add some new syntax that specifies that if a certain value can be narrowed into a tuple then we should do so:

// Arbitrary syntax 1
declare function zipLongest(
  iterables: [...tuple Ts]
): Iterable<ZipUnwrapped<Ts>>

// Arbitrary syntax 2
declare function zipLongest<Ts extends [!...Iterable<any>]>(
  iterables: Ts
): Iterable<ZipUnwrapped<Ts>>

The syntax isn't really all that important. The main downside of this approach is that we need two overloads, one for the narrowed tuple and one for the non-narrowed tuple. Additionally extra syntax would need to be supported and maintained.

Examples

Other than the above example another example that would benefit is the builtin lib.promise.d.ts which specifies overloads in this repetitive overloading pattern.

We could write something like Promise.all like so:

type UnwrapMaybePromiseLike<T>
= T extends PromiseLike<infer R> ? R : T

type UnwrappedArray<T extends Array<any>> = {
    [P in keyof T]: UnwrapMaybePromiseLike<T[P]>
}

interface PromiseConstructor {
    all<Ts extends Array<any>>(
        promises: [...Ts]
    ): UnwrappedArray<Ts>
}

And inference would just work.

// a would inferred to be [number, number]
const a = Promise.all([1, 2])

// b would be inferred to be [string, number]
const b = Promise.all(['foo', Promise.resolve(12)])

Checklist

My suggestion meets these guidelines:

• [?] This wouldn't be a breaking change in existing TypeScript / JavaScript code
• [✔] This wouldn't change the runtime behavior of existing JavaScript code
• [✔] This could be implemented without emitting different JS based on the types of the expressions
• [✔] This isn't a runtime feature (e.g. new expression-level syntax)

EDIT: Realized Promise.race was supposed to be Promise.all oops.

[dhruvrajvanshi]

You can force an array to be treated as a tuple by creating a function

function tuple<Args extends any[]>(...args: Args): Args { return Args; }

const pair = tuple(1, "string");
// pair is inferred as [number, string]

The rest of it seems a bit more tricky. Mapped types don't work properly with tuples so it might need language level support to convert one tuples type to another (sort of like array.map but for type level tuples)

An even more general approach might be to allow recursion in types so we can use conditional types to write our own mapping logic.

EDIT:

Apparently, your Promise.race example doesn't need any language changes.

interface PromiseConstructor {
    race<Ts extends Array<any>>(
        promises: Ts
    ): UnwrappedArray<Ts>
}

This works fine.

The first example works too using the tuple function.

declare function zipLongest<Ts extends Array<Iterable<any>>>(iterables: Ts): Iterable<ZipUnwrapped<Ts>>;
const strs = ['foo', 'bar', 'baz', 'boz']
const nums = [1, 2, 3, 4]
const zipPair = tuple(strs, nums);
const z = zipLongest(zipPair)

Basically, instead of doing [...Ts] you just write Ts.

[ahejlsberg]

I think you can already do this with features currently in typescript@next. In particular, the work in #26063 enables your scenario.

type Iterablized<T> = { [K in keyof T]: Iterable<T[K]> };

declare function zipLongest<T extends any[]>(iterables: Iterablized<T>): Iterable<T>;

declare let triple: [Iterable<string>, Iterable<number>, Iterable<string[]>];

let zippedTriple = zipLongest(triple);  // Iterable<[string, number, string[]>

[ahejlsberg]

With respect to explicit syntax for creating tuples from array literals, we're tracking and debating that in #16656.

Meanwhile, with #24897 (in TS 3.0) you can easily add the following convenience function:

function tuple<T extends any[]>(...elements: T) {
    return elements;
}

and you then have a very low cost general way of creating tuples. For example:

const t1 = tuple(1, "hello");      // [number, string]
const t2 = tuple(1, tuple(2, 3));  // [number, [number, number]]
const t3 = tuple(1, [2, 3]);       // [number, number[]]
const t4 = tuple();                // []

[Jamesernator]

I've been trying it with typescript@next and the mapped types isn't quite the problem, strictly speaking the definition above works, it's just not particularly useful when doing it inline with inferred types e.g.:

for (const [a, b] of zipLongest([strings, numbers]) {
  // Inferred type of both a and b are string | number
}

Which is what I'm proposing improving. Basically someway to say that if [strings, numbers] can be inferred as a tuple, it should be.

This already works (in typescript@next) when the argument is variadic rather than as an array directly e.g.:

type Iterablized<T> = { [K in keyof T]: Iterable<T[K] | undefined> };

declare function zipLongest<Ts extends Array<any>>(...iterables: Ts): Iterablized<Ts>

const strs = ['foo', 'bar', 'baz']
const nums = [1, 2, 3]
// Correctly inferred to be Iterable<[string | undefined, number | undefined]>
const z = zipLongest(strs, nums)

However this can't be used as I'm extending the zipLongest with additional arguments that can't be disambiguated from iterables. So it'll look something like this soon:

const fillers = [() => '', () => 0]
for (const [a, b] of zipLongest([strs, nums], fillers)) {

}

[ahejlsberg]

@Jamesernator One trick that does work is to ensure that the contextual type for the array literal includes a tuple type. Once that is the case we'll infer tuple types:

declare function zipLongest<Ts extends Iterable<any>[] | [Iterable<any>]>(iterables: Ts): Iterable<ZipUnwrapped<Ts>>;

[Jamesernator]

Oh that's useful, that should probably be added into the handbook somewhere as I don't think I would've ever figured that out.

[ahejlsberg]

@DanielRosenwasser @weswigham FYI since we were discussing this the other day. Note that if we put this in the handbook, we should recommend using T extends XXX[] | [XXX] as the constraint pattern (versus T extends XXX[] | []) because this pattern ensures that members on T are just what you would see on XXX[]. (With an empty tuple in the pattern some never types creep in.)

[typescript-bot]

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