concurrency - Are Futures in Scala really functional?

Question

I am reading this blog post that claims Futures are not "functional" since they are just wrappers of side-effectful computations. For instance, they contain RPC calls, HTTP requests, etc. Is it correct ?

The blog post gives the following example:

def twoUsersFeed(a: UserHandle, b: UserHandle)
                (implicit ec: ExecutionContext): Future[Html] =
  for {
    feedA <- usersFeed(a)
    feedB <- usersFeed(b)
  } yield feedA ++ feedB

you lose the desired property: consistent results (the referential transparency). Also you lose the property of making as few requests as possible. It is difficult to use multi-valued requests and have composable code.

I am afraid I don't get it. Could you explain how we lose the consistent result in this case ?

Answer 1

The blog post fails to draw a proper distinction between Future itself and the way it's commonly used, IMO. You could write pure-functional code with Future, if you only ever wrote Futures that called pure, total functions; such code would be referentially transparent and "functional" in every remotely reasonable sense of the word.

What is true is that Futures give you limited control of side effects, if you use them with methods that have side effects. If you create a Futurewrapping webClient.get, then creating that Future will send a HTTP call. But that's not a fact about Future, that's a fact about webClient.get!

There is a grain of truth in this blog post. Separating expressing your computation from executing it, completely, via e.g. the Free monad, can result in more efficient and more testable code. E.g. you can create a "query language", where you express an operation like "fetch the profile photos of all the mutual friends of A and B" without actually running it. This makes it easier to test if your logic is correct (because it's very easy to make e.g. a test implementation that can "run" the same queries - or even just inspect the "query object" directly) and, as I think the blog post is trying to suggest, means you could e.g. combine multiple requests to fetch the same profile. (This isn't even purely a functional-programming concern - some OO books have the idea of a "command pattern" - though IME functional programming tools like for/yield syntax make it much easier to work in this way). Whereas if all you have is a fetchProfile method that, when run, immediately fires off a HTTP request, then if your code logic requests the same profile twice, there's no way to avoid fetching the same profile twice.

But that isn't really about Future per se, and IMO this blog post is more confusing than helpful.