Support defining lengths of arrays in function parameters with mapped tuples

[Roaders]

Search Terms

mapped tuple

Suggestion

Support the use of mapped tuples to define the length of arrays passed to a function

Use Cases

The actual example that I working with currently is that I want to construct a list of items, I pass an array of constructors to a function and I want to also pass to the function a list of configuration objects for each constructor. I tried to do this with a mapped tuple but the constructor list is typed as an array rather than a tuple so the length is not honoured and configuration lists of any length are allowed by the compiler

Examples

const myStringArray = ["one", "two", "three"];

type MapToBoolean<T extends string[]> = { [K in keyof T]: boolean };

function mapStrings<T extends string[]>(strings: T, modifyStrings: MapToBoolean<T>) {
}

mapStrings(myStringArray, []); // should fail compilation
mapStrings(myStringArray, [true, true]); // should fail compilation
mapStrings(myStringArray, [true, true, true]); // should pass compilation
mapStrings(myStringArray, [true, true, true, true]); // should pass compilation

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)

[AlCalzone]

Duplicate of #26223?

[Roaders]

Duplicate of #26223?

I don't think so. I don't want to specify the length of the tuple, I want to support any length. I just want the length of the 2 arrays to be the same.

[jack-williams]

If you contextually type your tuple then you get some of the errors you want:

const myStringArray: [string, string, string] = ["one", "two", "three"];

type MapToBoolean<T extends string[]> = { [K in keyof T]: boolean };

function mapStrings<T extends string[]>(strings: T, modifyStrings: MapToBoolean<T>) {
}

mapStrings(myStringArray, []); // should fail compilation - Does
mapStrings(myStringArray, [true, true]); // should fail compilation - Does
mapStrings(myStringArray, [true, true, true]); // should pass compilation - Does
mapStrings(myStringArray, [true, true, true, true]); // should pass compilation - Does NOT. 4 is not assignable to 3

I'm not sure whether the last one should be ok. I think the error seems reasonable.

[Roaders]

@jack-williams - that would be fine if I could require a tuple type as the function argument and not accept arrays.

[jack-williams]

type VerifyTuple<T> =
    T extends { length: infer L } ?
    number extends L ? 'not a tuple' : T
    : 'not an array';

const myStringArray = ["one", "two", "three"];

type MapToBoolean<T extends string[]> = { [K in keyof T]: boolean };

function mapStrings<T extends string[]>(strings: T & VerifyTuple<T>, modifyStrings: MapToBoolean<T>) {

}

mapStrings(myStringArray, []); // should fail compilation
mapStrings(myStringArray, [true, true]); // should fail compilation
mapStrings(myStringArray, [true,true, true]); // should pass compilation
mapStrings(myStringArray, [true, true, true, true]); // should pass compilation

[typescript-bot]

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