Skip to content

Pragmatic yet more correct narrowing algorithm for UDTPΒ #47389

Closed
@devanshj

Description

@devanshj

Suggestion

πŸ” Search Terms

Narrowing, User defined typed predicate, first-filter narrowing

βœ… Viability Checklist

  • This wouldn't be a breaking change in existing TypeScript/JavaScript code (Could break some code, but only the code that is already buggy and can produce runtime errors)
  • 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, new syntax sugar for JS, etc.)
  • This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Let's take an example...

interface Dog { woof: () => void }
interface Cat { meow: () => void }
interface Fish { swim: () => void }  

declare const isCatOrFish: (x: unknown) => x is Cat | Fish
declare let catOrDog: Cat | Dog
if (isCatOrFish(catOrDog)) {
  catOrDog.meow()
  catOrDog;
// ^? Cat
}

Okay so first off all, by definition of "narrowing", the narrowed catOrDog should have been (Cat | Dog) & (Cat | Fish) and not Cat, both are not same. But TypeScript makes a good pragmatic decision to make it Cat, and how does it do that? Via an algorithm Ryan describes here. To describe it better let me convert that algorithm into code...

type TypeScriptNarrow
  < T // to-narrow
  , N // narrowed
  , ShouldBePragmatic =
      // if any of the narrowed constituent is a subtype of any of the to-narrow constituent
      ( N extends unknown
          ? N extends T ? true : false
          : never
      ) extends false
        ? false
        : true
  > = 
    ShouldBePragmatic extends true
      ? T extends unknown
          ? Extract<T, N> 
          : never
      : T & N

interface Dog { woof: () => void }
interface Cat { meow: () => void }
interface Fish { swim: () => void }  
interface Monkey { climb: () => void } 

type T0 = TypeScriptNarrow<Dog | Cat, Cat>
//   ^? Cat
type T1 = TypeScriptNarrow<Dog | Cat, Cat | Fish>
//   ^? Cat
type T2 = TypeScriptNarrow<Dog | Monkey, Cat | Fish>
//   ^? (Dog | Monkey) & (Cat | Fish)
type T3 = TypeScriptNarrow<{ a: string | undefined } | { b: string }, { a: string } | { b: string }>
//   ^? { b: string }

The first three cases are good, the last one is problematic. Consider this...

const foo = (x: { a: string | undefined } | { b: string }) => {
  if (areValuesDefined(x)) {
    console.log(x.b.toUpperCase())
  }
}

const areValuesDefined = <T>(x: T): x is { [K in keyof T]: Exclude<T[K], undefined> } =>
  Object.values(x).every(x => x !== undefined)

foo({ a: "hello" }) // TypeError: Cannot read properties of undefined (reading 'toUpperCase')

Here x gets narrowed to { b: string } (instead of something like { a: string } | { b: string }) because of the current algorithm and makes the type system unsound.

So instead of the current algorithm I propose this algorithm...

type DevanshNarrow
  < T // to-narrow
  , N // narrowed
  , ShouldBePragmatic =
      // if any of the narrowed constituent is a subtype of any of the to-narrow constituent
      ( N extends unknown
          ? N extends T ? true : false
          : never
      ) extends false
        ? false
        : true
  > = 
    ShouldBePragmatic extends true
      ? T extends unknown
          ? N extends unknown
              ? keyof T & keyof N extends never
                  ? never
                  : T & N
              : never
          : never
      : T & N

interface Dog { woof: () => void }
interface Cat { meow: () => void }
interface Fish { swim: () => void }  
interface Monkey { climb: () => void } 

type T0 = DevanshNarrow<Dog | Cat, Cat>
//   ^? Cat
type T1 = DevanshNarrow<Dog | Cat, Cat | Fish>
//   ^? Cat
type T2 = DevanshNarrow<Dog | Monkey, Cat | Fish>
//   ^? (Dog | Monkey) & (Cat | Fish)
type T3 = DevanshNarrow<{ a: string | undefined } | { b: string }, { a: string } | { b: string }>
//   ^? | ({ a: string | undefined } & { a: string })
//      | ({ b: string } & { b: string })

So the first three results remain the same, hence it's still pragmatic but now the last result is more correct with DevanshNarrow than TypeScriptNarrow. With this algorithm the unsound code above would not compile.

The idea is instead of an actual intersection, we do a pragmatic intersection that is if two types don't overlap their pragmatic intersection is never. And hence Dog <pragmaticIntersect> Cat becomes never

I wonder if there are cases where TypeScript's current algorithm yields better results than the proposed.

πŸ“ƒ Motivating Example

πŸ’» Use Cases

Metadata

Metadata

Assignees

No one assigned

    Labels

    Experimentation NeededSomeone needs to try this out to see what happensIn DiscussionNot yet reached consensusSuggestionAn idea for TypeScript

    Type

    No type

    Projects

    No projects

    Milestone

    No milestone

    Relationships

    None yet

    Development

    No branches or pull requests

    Issue actions