flatMap<A, B, C> function

FlatMaps over the Either type.

Transforms an Either<A, B> into an Either<A, C> by applying the given function fn to the inner value if it's of type Right<B>. If the Either is a Left<A>, it returns the same Left<A> without any modification.

Example:

Consider you have an Either that can be an integer error code or a valid string:

Either<int, String> name = Right("Alice");

You can use the flatMap function to derive another Either based on this value:

final result = flatMap<int, String, int>((s) => s == "Alice" ? Right(s.length) : Left(-1))(name);

if (result is Right<int>) {
  print("Name length is ${result.value}");
} else if (result is Left<int>) {
  print("Error code: ${result.value}");
}

In this example, since name is Right("Alice"), the output will be:

Name length is 5

If name was any other Right<String> value, the output would be:

Error code: -1

If name was a Left<int>, say Left(404), the output would be:

Error code: 404

@param fn The function to apply on the inner value. @return A function that takes in an Either<A, B> and returns an Either<A, C>.