I am a Scala programmer, learning Haskell now. It’s easy to find practical use cases and real world examples for OO concepts, such as decorators, strategy pattern etc. Books and interwebs are filled with it.
I came to the realization that this somehow is not the case for functional concepts. Case in point: applicatives.
I am struggling to find practical use cases for applicatives. Almost all of the tutorials and books I have come across so far provide the examples of [] and Maybe. I expected applicatives to be more applicable than that, seeing all the attention they get in the FP community.
I think I understand the conceptual basis for applicatives (maybe I am wrong), and I have waited long for my moment of enlightenment. But it doesn’t seem to be happening. Never while programming, have I had a moment when I would shout with a joy, “Eureka! I can use applicative here!” (except again, for [] and Maybe).
Can someone please guide me how applicatives can be used in a day-to-day programming? How do I start spotting the pattern? Thanks!
Warning: my answer is rather preachy/apologetic. So sue me.
Well, how often in your day-to-day Haskell programming do you create new data types? Sounds like you want to know when to make your own Applicative instance, and in all honesty unless you are rolling your own parser, you probably won’t need to do it very much. Using applicative instances, on the other hand, you should learn to do frequently.
Applicative is not a “design pattern” like decorators or strategies. It is an abstraction, which makes it much more pervasive and generally useful, but much less tangible. The reason you have a hard time finding “practical uses” is because the example uses for it are almost too simple. You use decorators to put scrollbars on windows. You use strategies to unify the interface for both aggressive and defensive moves for your chess bot. But what are applicatives for? Well, they’re a lot more generalized, so it’s hard to say what they are for, and that’s OK. Applicatives are handy as parsing combinators; the Yesod web framework uses Applicative to help set up and extract information from forms. If you look, you’ll find a million and one uses for Applicative; it’s all over the place. But since it’s so abstract, you just need to get the feel for it in order to recognize the many places where it can help make your life easier.