Type Alias Widen<A, B>

Widen: A extends B ? B : never

The TypeScript equivalent of Scala's B >: A (B is a supertype of A).

Resolves to B when A extends B (i.e., B is a legitimate supertype of A, or B = A). Resolves to never otherwise, which renders any callback position using Widen uncallable — the user gets a compile-time error rather than a runtime type lie.

Type Parameters

Example

// Inside a container interface:
reduce<B = A>(op: (b: Widen<A, B>, a: Widen<A, B>) => Widen<A, B>): Widen<A, B>

// Callers:
list.reduce((a, b) => a + b)          // B defaults to A, Widen<A,A> = A, fine
list.reduce<A | "extra">(...)         // A extends A | "extra", Widen = B, fine
list.reduce<UnrelatedType>(...)       // A doesn't extend Unrelated, Widen = never, compile error

Type Alias Widen<A, B>

Type Parameters

Example

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