TypeScript tuple inference

tuple.md

You can use the tuple() function in tuple.ts to infer tuple types in TypeScript and cut down on the need to repeat yourself. Without tuple(), declaring a constant of a tuple type looks like this:

const daysOfTheWeek: ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"] = 
  ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"];

You can't do this:

const daysOfTheWeek = ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"]; 
  // right value, wrong type

because TypeScript would infer the type of daysOfTheWeek to be a plain array of string values.

And you can't do this:

const daysOfTheWeek: ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"]; 
  // right type, wrong value

because then the variable daysOfTheWeek would not be defined at runtime.

Enter tuple():

import { tuple } from './tuple'; // or wherever you put your libraries
const daysOfTheWeek = 
  tuple("sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"); // right type, right value

If you inspect the type of daysOfTheWeek, it will be the desired tuple type ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"], and has the correct value at runtime.

---

Note that the inferred tuple type will be as narrow as it can, interpreting any string, number, or boolean literal element as the corresponding literal type:

const narrow = tuple("a", 1, true); // inferred as type ["a", 1, true]

If you want to widen any of them, you can do type assertions:

const wider = tuple("a" as string, 1 as 0 | 1 | 2, true as boolean | undefined) 
// inferred as [string, 0 | 1 | 2, boolean | undefined]

Also note that the library only supports tuples of length up to twelve; you can add more overloads at the top if you need them.

tuple.ts

export type Lit = string | number | boolean | undefined | null | void | {};

// infers a tuple type for up to twelve values (add more here if you need them)
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit, I extends Lit, J extends Lit, K extends Lit, L extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J, k: K, l: L): [A, B, C, D, E, F, G, H, I, J, K, L];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit, I extends Lit, J extends Lit, K extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J, k: K): [A, B, C, D, E, F, G, H, I, J, K];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit, I extends Lit, J extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J): [A, B, C, D, E, F, G, H, I, J];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit, I extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I): [A, B, C, D, E, F, G, H, I];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H): [A, B, C, D, E, F, G, H];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G): [A, B, C, D, E, F, G];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F): [A, B, C, D, E, F];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit>(a: A, b: B, c: C, d: D, e: E): [A, B, C, D, E];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit>(a: A, b: B, c: C, d: D): [A, B, C, D];
export function tuple<A extends Lit, B extends Lit, C extends Lit>(a: A, b: B, c: C): [A, B, C];
export function tuple<A extends Lit, B extends Lit>(a: A, b: B): [A, B];
export function tuple<A extends Lit>(a: A): [A];
export function tuple(...args: any[]): any[] {
  return args;
}

We're getting very close to not needing this at all... In TypeScript 3.4, which should be released in March 2019 it will be possible to tell the compiler to infer the type of a tuple of literals as a tuple of literals by using the as const syntax. It should look like this:

const list = ['a', 'b', 'c'] as const;  // const list: readonly ['a', 'b', 'c']

@jcalz

using

export type Lit = string | number | boolean | undefined | null | void | {};
export const tuple = <T extends Lit[]>(...args: T) => args;

and invoking tuple('string1', 'string2') gives me error TS2370: A rest parameter must be of an array type (presumably because I'm passing in 4 seperate string arguments, not an array it can destructure?)

as const is nice!

@laurencestokes I can't reproduce your problem, it works in playground.

as const was perfect. some link to the documentation to know more?

as const was perfect. some link to the documentation to know more?

This feature was added in TS 3.4