What are practical uses of applicative style?

Question

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 an

Answer 1

I think of Functor, Applicative and Monad as design patterns.

Imagine you want to write a Future[T] class. That is, a class that holds values that are to be calculated.

In a Java mindset, you might create it like

trait Future[T] {
  def get: T
}

Where 'get' blocks until the value is available.

You might realize this, and rewrite it to take a callback:

trait Future[T] {
  def foreach(f: T => Unit): Unit
}

But then what happens if there are two uses for the future? It means you need to keep a list of callbacks. Also, what happens if a method receives a Future[Int] and needs to return a calculation based on the Int inside? Or what do you do if you have two futures and you need to calculate something based on the values they will provide?

But if you know of FP concepts, you know that instead of working directly on T, you can manipulate the Future instance.

trait Future[T] {
  def map[U](f: T => U): Future[U]
}

Now your application changes so that each time you need to work on the contained value, you just return a new Future.

Once you start in this path, you can't stop there. You realize that in order to manipulate two futures, you just need to model as an applicative, in order to create futures, you need a monad definition for future, etc.

UPDATE: As suggested by @Eric, I've written a blog post: http://www.tikalk.com/incubator/blog/functional-programming-scala-rest-us