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

/* tslint:disable:max-line-length */

// TypeScript wizardry thanks to jcalz:
// https://stackoverflow.com/a/45486495/14379
// https://gist.github.com/jcalz/381562d282ebaa9b41217d1b31e2c211
// Readonly added by seanf: https://gist.github.com/seanf/b056a40a7256efee10f53c82e490b5b3
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] & Readonly<[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] & Readonly<[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] & Readonly<[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] & Readonly<[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] & Readonly<[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] & Readonly<[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] & Readonly<[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] & Readonly<[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] & Readonly<[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] & Readonly<[A, B, C]>;
export function tuple<A extends Lit, B extends Lit>(a: A, b: B): [A, B] & Readonly<[A, B]>;
export function tuple<A extends Lit>(a: A): [A] & Readonly<[A]>;
export function tuple(...args: any[]): Readonly<any[]> {
  return args;
}