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Support open-ended unions #26277

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@ghost

Description

Suggestion

Ability to discriminate between union members when not all union members are known.

Use Cases

  • redux actions: See Allow switch type guards #2214 (comment)
  • This would enable us to discriminate on Node kinds without converting Node to a huge union (which slows down compilation too much). (Also, Node is effectively open-ended because we add new kinds fairly often.)

Examples

interface Shape {
    unique kind: string;
}

interface Square extends Shape {
    kind: "square";
    size: number;
}

interface Circle extends Shape {
    kind: "circle";
    radius: number;
}

// other shapes may exist

function area(s: Shape) {
    switch (s.kind) {
        case "square":
            return s.size * s.size;
        case "circle":
            return Math.PI * s.radius ** 2;
        default:
            return 0; // Or hand off to some other function that handles other Shape kinds
    }
}

Checklist

My suggestion meets these guidelines:

  • This wouldn't be a breaking change in existing TypeScript / JavaScript code
  • This wouldn't change the runtime behavior of existing JavaScript code
  • This could be implemented without emitting different JS based on the types of the expressions
  • This isn't a runtime feature (e.g. new expression-level syntax)

Workarounds

Cast the general type to a union of known type.

function area(sIn: Shape) {
    const s = sIn as Square | Circle;
    ...

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