Repository navigation
Allow use of spread operator in tuple types #24902
Description
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 numbersCurrently, 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)
Activity
j-oliveras commentedon Jun 12, 2018
I think is already in progress in #24897 (see Open-ended tuple types block).
Anders Hejlsberg (@ahejlsberg) can answer better.
aneilbaboo commentedon Jun 12, 2018
Ah, thanks for the pointer, Jordi Oliveras Rovira (@j-oliveras).
Anders Hejlsberg (@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 commentedon Jun 12, 2018
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 commentedon Jun 12, 2018
Should be implemented by #24897. the syntax will be
[number, string*]Cryrivers commentedon Jun 13, 2018
UPDATE: final syntax will be
[number, ...string[]].