In this chapter, we have seen how we can model the possibility of an optional value with Option and the possibility of an error with Either. We demonstrated how these types can replace exceptions while guaranteeing referential transparency.
We also saw how we can combine several Option or Either types using flatMap. This works well when we have to check for optional values or errors sequentially—call function1; if there is no error, call function2; if there is no error, call function3. If any of these functions return an error, we return that error and stop the call chain, as shown in the following code:
def sequentialErrorHandling(x: String): Either[MyError, String] =
for {
a <- function1(x)
b <- function2(a)
c <- function3(b)
} yield cHowever, in some situations, we would want to call several functions in parallel and return all the errors that might have occurred. For instance, when you enter some personal details to purchase a product from an...