[SOLVED] Reversed flatMap

Question
1

As I understand it, flatMap - for example on Either passes-through Lefts and applies the mapping on the value of Rights.

Now, I wonder, if the reversed operation exists, too: Passing-through the Rights but applying the mapping on the value of Lefts.

Usecase: Given a method that returns an Either[BackendError, Value]. The result of this method should be mapped into an Either[WrappedBackendError, Value].

What I do now is:

val result: Either[BackendError, Value] = someMethod
result match {
  case Left(error) => Left(WrappedBackendError(error))
  case Right(value) => Right(value)
}
2

Either has method .swap which swaps right with left so that you can use the right-biased flatMap.

3

You mean something like that?

val result: Either[BackendError, Value] = someMethod
result.swap flatMap { error => 
  WrappedBackendError(error)
}.swap
4

Yeah, that is the stdlib way of solving that problem.

If you are using *cats, you can:

result.leftFlatMap(error => Left(WrappedBackendError(error)))

Or more generally:

result.handleErrorWith(error => Left(WrappedBackendError(error)))

However, it seems you really only need to:

result.leftMap(error => WrappedBackendError(error))
5

In the std lib, you can also go through the left projection: result.left.map(...).

1 Like
6

Isn’t that deprecated?
Genuine question, I really do not understand if it was deprecated or not.

7

There seems to have been some back and forth on that subject, but as of 2.13.2, left projection doesn’t seem to be deprecated.

1 Like
8

Thakn you for the clarification!