Allow use of spread operator in tuple types

[aneilbaboo]

Search Terms

tuple type rest spread typescript

Suggestion

I'd like to be able to write tuple types using the spread operator. For example:

type Foo = [string, ...number[]]; // first elt must be string, rest must be numbers

Currently, this produces an error at the ellipsis: "Type expected".

Here is a question asking much the same thing on Stack Overflow: https://stackoverflow.com/questions/44934541/how-to-use-the-spread-operator-in-a-typescript-tuple

It seems related to #10727, but that issue focuses on Object rest/spread, whereas this is specifically about tuples.

Use Cases

Typed tuples are part of the existing language. So it seems natural that one should be able to type rest arguments .

For example, if you want to represent a syntax tree, you might want to write:

type FunCall = ['FUNCALL', Symbol, ...Expression[]]; // error in 2.9.1 :(
type Symbol = string;
type Expression = FunCall | Scalar;
type Scalar = number | string | null;

But this isn't allowed in Typescript 2.9.1.

Examples

type Foo = [string, ...number[]];
const f1: Foo = ["str", 2, 3, 4]; // should pass
const f2: Foo = ["str", "2", 3, 4]; // should fail: 
		// "Type '(string | number)[]' is not assignable to type 'number[]'.
const f2: Foo = ["str", 2, "3", 4]; // should fail:
		// "Type '(number | string)[]' is not assignable to type 'number[]'.
const f1: Foo = [1, 2, 3, 4]; // should fail:
		// "Type 'number' is not assignable to type 'string'.

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)

[j-oliveras]

I think is already in progress in #24897 (see Open-ended tuple types block).

@ahejlsberg can answer better.

[aneilbaboo]

Ah, thanks for the pointer, @j-oliveras.

@ahejlsberg, #24897 seems to deal with rest arguments to functions, whereas this deals with definition of tuple types with the rest operator. Does it makes sense to include tuple types in that issue?

[aneilbaboo]

Here's another issue which focuses on rest in function calls: #17934

And a work in progress which uses generics for defining tuple types: #17884

[mhegazy]

Should be implemented by #24897. the syntax will be [number, string*]

[Cryrivers]

UPDATE: final syntax will be [number, ...string[]].