Feature: Composing spread operator on Union types and static_key function to enable generic capability

Related issues: #13542 #15759 (slightly different take and example on closed issues)

I originally posted on issue #15759 but decided that they were different enough to be looked at separately and that issue doesn't address a generic capability. Look back at https://github.com/Microsoft/TypeScript/issues/15759#issuecomment-314157616 for clunky workaround examples.

Proposal

1. Add spread operator ... to string literal union types acting as an string array or generator. Type T applying spread operator will result in type T[]. For example:

type Group = 'red' | 'blue' | 'green';
const groups = ...Group; // groups has type of Group[] with values ['red', 'green', 'blue']

2. Add static function static_key or keyof that transforms a static interface to a string literal union of all combined types. For example:

interface Groups<T, U, V> {
  red: T;
  green: U;
  blue: V;
}
type groups = static_key(Groups); // 'red' | 'blue' | 'green'

3. Composing both can result in powerful generic operations. I feel that this could bring a unique capability that string enums could not and may have been overlooked. It would be very powerful for a Record type or interface where all fields are the same type. Here is an example:

interface GroupDescription {
    name: string;
    description: string;
}

type Group = 'red' | 'blue' | 'green';
type Groups = { [K in Group]: GroupDescription };

function initGroups(groups: Groups) {
    for(const group of ...static_key(Groups)) { // group would be type Group or 'red' | 'blue' | 'green'
        processGroup(name); //Generically Process property
    }
}

function processGroup(group: Group) { }

I noticed I don't need ...static_key but instead directly use the spread operator on the union type ...Group. The point is that they compose well together and they add generic capability.

L’issue non indica file, test o punti di ingresso. Inizia leggendo la proposta e le issue correlate #13542 e #15759, incluso il commento collegato sul workaround, quindi definisci un design condiviso per il union spreading e static_key. Per considerare il lavoro completato, sono necessari l’ambito e i criteri di implementazione approvati da un maintainer.