Either altW 'left' is too wide

[wmaurer]

🐛 Bug report

The function signature for Either altW is as follows:

export declare const altW: <E2, B>(that: Lazy<Either<E2, B>>) => <E1, A>(fa: Either<E1, A>) => Either<E1 | E2, A | B>

From my understanding of alt, Left<E1> can never be produced, so I believe the signature should be as follows:

export declare const altW: <E2, B>(that: Lazy<Either<E2, B>>) => <E1, A>(fa: Either<E1, A>) => Either<E2, A | B>

Current Behavior

const foo = pipe(
    E.left<number, number>(1),
    E.altW(() => E.right<string, string>('a')),
);

In this case, foo's type is Either<string | number, string | number>.

Expected behavior

I believe foo's type should be Either<string, string | number>.

Reproducible example

See above.

Suggested solution(s)

Remove E1 from the return type of altW:

export const altW: <E2, B>(
    that: Lazy<Either<E2, B>>,
) => <E1, A>(fa: Either<E1, A>) => Either<E2, A | B> = that => fa => (isLeft(fa) ? that() : fa);

I could make a PR if you agree.

Your environment

Software	Version(s)
fp-ts	2.10.2
TypeScript	4.2.4

[gcanti]

I could make a PR if you agree.

@wmaurer thanks, you are right the current error type is overspecified. Looks like the following data types suffer from the same issue:

• Either
• IOEither
• ReaderEither
• ReaderTaskEither
• StateReaderTaskEither
• TaskEither

[wmaurer]

Great, thanks for the confirmation. I'll put together a PR soon.