Improve types of Result.{isOk, isErr, value}

[ysulyma]

Improve types of Result.{isOk, isErr, value}

This improves type narrowing as shown below:

const x = {} as unknown as Result<string, number>;

const y = x.value(); // string | number

if (x.isOk()) {
  const y = x.value(); // string
} else {
  const y = x.value(); // number
}

if (x.isErr()) {
  const y = x.value(); // number
} else {
  const y = x.value(); // string
}

• I have added or updated unit tests for this change. (N/A)
• I have updated the changelog with info about the changes in this PR

[ysulyma]

hmm actually this breaks some existing code. I think the library should do this, but it might require more significant changes to the types...

[rametta]

Thanks for the PR @ysulyma. I've been meaning to improve the type narrowing for awhile. Ideally we would use Higher Kinded Types (HKT's) for this. I have another library which I have used that concept before and it works flawlessly (https://github.com/rametta/srd) feel free to take a look, that concept can be applied here too