Description
Search Terms
Suggestion
Right now, even with strictNullChecks is set to true, null cannot be used as a discriminator for tagged union.
Use Cases
Recently, I made a pull request for @types/node in which I let spawnSync returns a tagged union with error property as discriminator.
Examples
interface WithError {
error: Error
data: null
}
interface WithoutError<Data> {
error: null
data: Data
}
type DataCarrier<Data> = WithError | WithoutError<Data>
// MAIN FOCUS:
// TypeScript should be able to deduce error's and data's
// types in the following function
function test<Data>(carrier: DataCarrier<Data>) {
if (carrier.error === null) { // `error` property as disciminator
const error: null = carrier.error // should work
const data: Data = carrier.data // should work
} else {
const error: Error = carrier.error // should work
const data: null = carrier.data // should work
}
}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)