For a Result[T], FlatMap should apply f when the result is Ok, and
propagate the error unchanged when it's Err. The original code returns r
on Ok and tries to use r.left when Err, which is wrong. This commit
fixes that.
Secondly, the group of FlatMap/AndThen and OrElse functions and methods
are now properly tested with new unit tests.
fixes#10401
`NewResult` makes it easy to wrap a normal function call in a result
value.
`Err` can be used to check the error case of the result without
unpacking the entire thing.
`AndThen2` allows a caller to compose a function on two results values,
with the closure only executing if both values are non-error.
In this commit, we add a new result type, which is just Either[A, error]. This is useful as we can always pass around/accept a single value. We add an Unpack() method which is the most idiomatic way of handling errors in Go, as we revert back to (A, error) when necessary.
Gijs van Dam