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Now we see
flatMap are an important trio of operations and each does one thing and does it well. We should now be able to convince ourselves that we shouldn’t be smudging their definitions just to suit our needs. We will likely come across functions with signatures that look a lot like
flatMap and may even be tempted to call it
flatMap, but doing so can destroy all of our intuitions around what
flatMap is. Right now we have 6 types that we are very familiar with and all of the operations behave roughly the same, even for very different types.
And this lesson is an important one, but just 10 or 11 months ago we had a decisive moment in Swift history where the community got to learn from this and put it to real-world use. We actually had an entire episode dedicated to this back then, but now we are even in a better position to appreciate it, so let’s briefly recall the problem.
Apple’s Swift NIO project has a type
EventLoopFuture that can be thought of as a super charged version of the
Parallel type we’ve used many times on this series. It comes with a method that has the same signature as
flatMap, but originally it was named
then. This pull-request renames the method to
flatMap, which brings it inline with the naming for
Result in the standard libary.
The Swift evolution review of the proposal to add a
Result type to the standard library. It discussed many functional facets of the
Result type, including which operators to include (including
flatMap), and how they should be defined.
This talk explains a nice metaphor to understand how
flatMap unlocks stateless error handling.
When you build real world applications, you are not always on the “happy path”. You must deal with validation, logging, network and service errors, and other annoyances. How do you manage all this within a functional paradigm, when you can’t use exceptions, or do early returns, and when you have no stateful data?
This talk will demonstrate a common approach to this challenge, using a fun and easy-to-understand “railway oriented programming” analogy. You’ll come away with insight into a powerful technique that handles errors in an elegant way using a simple, self-documenting design.
Up until Swift 4.1 there was an additional
flatMap on sequences that we did not consider in this episode, but that’s because it doesn’t act quite like the normal
flatMap. Swift ended up deprecating the overload, and we discuss why this happened in a previous episode:
Swift 4.1 deprecated and renamed a particular overload of
flatMap. What made this
flatMapdifferent from the others? We’ll explore this and how understanding that difference helps us explore generalizations of the operation to other structures and derive new, useful code!