TypeScript Variance Issue with `never` Type in Function Signature
Hey, sorry if its a stupid question, but I think I'm confused with either variance in TS or assignability of
error:
nevernever type. How come in the following snippet, the first call to fnfn with error type specified as nevernever does not work, but second call to fnfn with error type instantiated as ErrorError passes?interface Left<E> {
readonly _tag: 'left'
readonly value: E
}
interface Right<A> {
readonly _tag: 'right'
readonly value: A
}
type Either<A, E> = Right<A> | Left<E>
declare function right<A, E = never>(value: A): Either<A, E>
class Something<A, E> {
readonly _tag = 'Something'
constructor(
readonly task: () => Either<A, E>,
readonly onFinish: (e: Either<A, E>) => void,
) {}
}
declare function fn<T extends Something<any, any>>(x: T): T
fn(
new Something(
(): Either<number, never> => right(123),
(_) => {},
),
)
fn(
new Something(
(): Either<number, Error> => right(123),
(_) => {},
),
)interface Left<E> {
readonly _tag: 'left'
readonly value: E
}
interface Right<A> {
readonly _tag: 'right'
readonly value: A
}
type Either<A, E> = Right<A> | Left<E>
declare function right<A, E = never>(value: A): Either<A, E>
class Something<A, E> {
readonly _tag = 'Something'
constructor(
readonly task: () => Either<A, E>,
readonly onFinish: (e: Either<A, E>) => void,
) {}
}
declare function fn<T extends Something<any, any>>(x: T): T
fn(
new Something(
(): Either<number, never> => right(123),
(_) => {},
),
)
fn(
new Something(
(): Either<number, Error> => right(123),
(_) => {},
),
)error:
TS2345: Argument of type 'Something<number, never>' is not assignable to parameter of type 'Something<any, any>'.
Types of property 'onFinish' are incompatible.
Type '(e: Either<number, never>) => void' is not assignable to type '(e: Either<any, any>) => void'.
Types of parameters 'e' and 'e' are incompatible.
Type 'Either<any, any>' is not assignable to type 'Either<number, never>'.
Type 'Left<any>' is not assignable to type 'Either<number, never>'.
Type 'Left<any>' is not assignable to type 'Left<never>'.
Type 'any' is not assignable to type 'never'.
25 new Something(
~~~~~~~~~~~~~~
26 (): Either<number, never> => right(123),
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
27 (_) => {},
~~~~~~~~~~~~~~
28 ),
~~~TS2345: Argument of type 'Something<number, never>' is not assignable to parameter of type 'Something<any, any>'.
Types of property 'onFinish' are incompatible.
Type '(e: Either<number, never>) => void' is not assignable to type '(e: Either<any, any>) => void'.
Types of parameters 'e' and 'e' are incompatible.
Type 'Either<any, any>' is not assignable to type 'Either<number, never>'.
Type 'Left<any>' is not assignable to type 'Either<number, never>'.
Type 'Left<any>' is not assignable to type 'Left<never>'.
Type 'any' is not assignable to type 'never'.
25 new Something(
~~~~~~~~~~~~~~
26 (): Either<number, never> => right(123),
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
27 (_) => {},
~~~~~~~~~~~~~~
28 ),
~~~
