Boolean literal types and return type propagation for generators · Issue #2983 · microsoft/TypeScript (original) (raw)

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Description

This suggestion has a few pieces:

1. Implement boolean literal types for true and false, in a fashion similar to Singleton types under the form of string literal types #1003
2. Implement type guards for booleans. Essentially, control flow constructs like if-else, while, for, etc would be subject to a type guard if their guard expression is a boolean.
3. Now we can update generators to have a next method that returns { done: false; value: TYield; } | { done: true; value: TReturn; }, where TYield is inferred from the yield expressions of a generator, and TReturn is inferred from the return expressions of a generator.
4. Iterators would return { done: false; value: TYield; } | { done: true; value: any; } to be compatible with generators.
5. for-of, spread, array destructuring, and the type yielded by yield* would only pick the value associated with done being false.
6. The value of a yield* expression would pick the value associated with done being true.
7. Introduce a Generator type that can be used to track the desired type of a yield expression. This would be TNext, and would be the type of the parameter for the next method on a generator.

The generator type would look something like this:

interface Generator<TYield, TReturn, TNext> extends IterableIterator { next(value?: TNext): IteratorYieldResult | IteratorReturnResult; throw(exception: any): IteratorYieldResult | IteratorReturnResult; return(value: any): IteratorYieldResult | IteratorReturnResult; Symbol.iterator: Generator<TYield, TReturn, TNext>; [Symbol.toStringTag]: string; }