Permit functions that return a value to also serve as a type guard

Search Terms

Linear type, affine type, type guard

Suggestion

It would be very helpful to allow a function to serve as a type guard, but also return an unrelated value.

Use Cases

This can be used to express type changes as a result of mutating operations, covering some of the use cases of e.g. Rust's affine types. (See also #16148.)

Examples

Consider this example, compiled with --strictNullChecks:

type NonEmptyArray<T> = {
  pop(): T;
} & Array<T>;

function isNonEmpty<T>(array: Array<T>): array is NonEmptyArray<T>;
function isNonEmpty(array: Array<unknown>): boolean {
  return array.length > 0;
}

let array: string[] = ['element'];
if (isNonEmpty(array)) {  // Guard gives 'array' type NonEmptyArray<string>.
  const elem1: string = array.pop();  // Works. This is correct.
  const elem2: string = array.pop();  // Also works, but elem2 will be undefined at runtime!
}

We could make this correct if pop() could both return a value and behave as a type guard. This isn't great syntax, but nonetheless consider if this was supported:

type NonEmptyArray<T> = {
  pop(): T && this is Array<T>;
} & Array<T>;

function isNonEmpty<T>(array: Array<T>): array is NonEmptyArray<T>;
function isNonEmpty(array: Array<unknown>): boolean {
  return array.length > 0;
}

let array: string[] = ['element'];
if (isNonEmpty(array)) {  // Guard gives 'array' type NonEmptyArray<string>.
  const elem1: string = array.pop();  // Returns a string and gives 'array' type Array<string>.
  const elem2: string = array.pop();  // Doesn't compile; pop() returns 'string | undefined'!
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code

This can use a new, previously invalid syntax to avoid affecting any existing program.

• 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. library functionality, non-ECMAScript syntax with JavaScript output, etc.)

There's no change in the code that's emitted; this feature would exist purely at the level of the type system.

• This feature would agree with the rest of TypeScript's Design Goals.

I believe that it would. It seems to be aligned well, in particular, with "Statically identify constructs that are likely to be errors."

L'issue non indica alcun file sorgente, test o punto di ingresso del compilatore. Inizia individuando la gestione da parte di TypeScript dei predicati di tipo e dei tipi restituiti, quindi definisci la sintassi accettata e il comportamento di narrowing, incluso l'esempio mutante di pop(), con test del compilatore che coprano i casi nuovi e non validi.