Enforce that an array is exhaustive over a union type

Question

Given a strongly-typed tuple created using a technique such as described here:

const tuple = (...args: T) => args;
const furnitu         


        

Answer 1

There are ways of doing this but it might be a bit messy for you. The two stumbling blocks here are the absence of partial type parameter inference and invalid types. Here is my solution:

type Furniture = 'chair' | 'table' | 'lamp' | 'ottoman';

const exhaustiveStringTuple = () =>
  (
    ...x: L & ([T] extends [L[number]] ? L : [
      Error, "You are missing ", Exclude])
  ) => x;

const missingFurniture = exhaustiveStringTuple()('chair', 'table', 'lamp');
// error, [string, string, string] is not assignable to parameter of type
// ["chair", "table", "lamp"] & [Error, "You are missing", "ottoman"]

const extraFurniture = exhaustiveStringTuple()(
    'chair', 'table', 'lamp', 'ottoman', 'bidet');
// error, "bidet" is not assignable to a parameter of type 'Furniture'

const furniture = exhaustiveStringTuple()('chair', 'table', 'lamp', 'ottoman');
// okay

As you can see, exhaustiveStringTuple is a curried function, whose sole purpose is to take a manually specified type parameter T and then return a new function which takes arguments whose types are constrained by T but inferred by the call. (The currying could be eliminated if we had proper partial type parameter inference.) In your case, T will be specified as Furniture. If all you care about is exhaustiveStringTuple(), then you can use that instead:

const furnitureTuple = 
  (
    ...x: L & ([Furniture] extends [L[number]] ? L : [
    Error, "You are missing ", Exclude])
  ) => x;

const missingFurniture = furnitureTuple('chair', 'table', 'lamp');
// error, [string, string, string] is not assignable to parameter of type
// ["chair", "table", "lamp"] & [Error, "You are missing", "ottoman"]

const extraFurniture = furnitureTuple('chair', 'table', 'lamp', 'ottoman', 'bidet');
// error, "bidet" is not assignable to a parameter of type 'Furniture'

const furniture = furnitureTuple('chair', 'table', 'lamp', 'ottoman');
// okay

The other issue is that the error you get when you leave out a required argument is