Symbols as literal types

Consider following use case for string literals at the moment in Flux:

interface IAction {
    actionType: string
}
interface ISayHelloAction extends IAction {
    actionType: "SAY_HELLO";
}

// Somewhere in your code you check that the message is ISayHelloAction from that actionType string using normal if

function processActions(action: IAction|ISayHelloAction|...) {
    if (action.actionType === "SAY_HELLO") {
        // action is ISayHelloAction
    }
}

(Above example assumes #6028 gets fixed, for the moment one can implement own type guard.)

But it would be even better if one could use Symbol for the same thing:

interface IAction {
    actionType: Symbol
}
const SAY_HELLO: Symbol = Symbol();

interface ISayHelloAction extends IAction {
    actionType: SAY_HELLO; // "Symbol literal type"
}

function processActions(action: IAction|ISayHelloAction|...) {
    if (action.actionType === SAY_HELLO) {
        // ... is ISayHelloAction
    }
}

These symbol literal types has same type guard requirement as literal types.

FYI there is some previous discussion about symbols as literal types starting here.

Would be useful.

Here's another example where this would be useful:

export const __skip__ = Symbol('skip');

function filterMapArray<ValueType,ReturnType>(iterable: Iterable<ValueType>, callback: (v: ValueType, k: number) => ReturnType|symbol): ReturnType[] {
    let accum = [];

    let i = 0;
    for(let x of iterable) {
        let y = callback(x, i++);
        if(y !== __skip__) {
            accum.push(y);
        }
    }

    return accum; // TS2322: Type '(symbol | ReturnType)[]' is not assignable to type 'ReturnType[]'.
  Type 'symbol | ReturnType' is not assignable to type 'ReturnType'. Type 'symbol' is not assignable to type 'ReturnType'.
}

This code won't compile because I can only typehint against symbol, but not __skip__. TS can't determine that accum contains only ReturnTypes.

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Symbols as literal types #7436

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Symbols as literal types #7436

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yortus commented on Mar 9, 2016

Artazor commented on Mar 14, 2016

mnpenner commented on Jun 19, 2017

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