# Practical Future[Option[A]] in Scala

### Motivation

In real world concurrent code you often come across the `Future[Option[A]]`

type(where `A`

is usually some concrete type). And then you need to compose these things. This is not straightforward in Scala.

If you’ve done some Haskell just `import scalaz._`

and you can skip the rest of this article. Scalaz library defines a monad typeclass(and many others) that formally specifies what it means to be a monad(not “has a flatMap-ish thingy”). Then it’s easy to build abstractions upon this.

But what if you don’t want to add another dependency and just want to make this tiny bit more practical? I can be done and is not that complicated. You will also learn something and maybe even become motivated to bite the bullet and start using Scalaz.

### Meaning

The `Future[Option[A]]`

type combines the notion of concurrency(from `Future`

) and the notion of failure(from `Option`

) giving you a *concurrent computation that might fail*. What we want is a monad instance for this. With only standard library code you probably would use `Future#flatMap`

in combination with `Option#map`

and `Option#getOrElse`

(or just `Option#fold`

). This gets messy and unreadable quite quickly due to loads of boilerplate. Let’s fix this!

### Newtyping

A monad in scala means a `flatMap`

method, so it’s safe to assume we need to define a `flatMap`

with this special semantics somewhere. You might want to define an implicit class that has the new method *but* it wouldn’t work as `Future`

already has a `flatMap`

method. I took a page from Haskell’s book. When you want to override semantics there you wrap up the type into a *newtype*. This is just compile-time type information that is completely free at runtime. Luckily Scala has this in form of `AnyVal`

.

```
import concurrent.Future
case class FutureO[+A](future: Future[Option[A]]) extends AnyVal
```

I’ve also made it covariant for ease of use.

### The Monad

It’s actually quite easy to implement

import concurrent.{Future, ExecutionContext}

```
case class FutureO[+A](future: Future[Option[A]]) extends AnyVal {
def flatMap[B](f: A => FutureO[B])
(implicit ec: ExecutionContext): FutureO[B] = {
FutureO {
future.flatMap { optA =>
optA.map { a =>
f(a).future
} getOrElse Future.successful(None)
}
}
}
def map[B](f: A => B)
(implicit ec: ExecutionContext): FutureO[B] = {
FutureO(future.map(_ map f))
}
}
```

You need to pull in an execution context and then do the usual boilerplate thing ending up with a wrap again. I’ve also added `map`

method which is trivial but we need it because in Scala for comprehensions are desugared into `flatMap`

and `map`

to avoid using `point`

(abstract constructor) since it’s harder to express OO-style..

### Usage

What good is this `FutureO`

? Let’s do a contrived example. We need a function that might fail - `divideEven`

that only divides even numbers. And we need concurrency - we’ll divide two numbers concurrently.

```
def divideEven(n: Int): Option[Int] =
if (n % 2 == 0) Some(n/2) else None
//first spawn both computations
val f1 = Future(divideEven(14))
val f2 = Future(divideEven(16))
//and combine them
val fc = for {
a <- FutureO(f1)
b <- FutureO(f2)
} yield a + b
//prints out Success(Some(15))
fc.future onComplete println
```

It works. How would this look without `FutureO`

?

```
val fc = for {
oa <- f1
ob <- f2
} yield for {
a <- oa
b <- ob
} yield a + b
fc onComplete println
```

Two layers of for comprehensions. It gets even hairier if you have data dependencies between your futures. Consider `divideEven`

again but this time we want to divide a number twice in a row. And we’ll be keeping the futures around just to prove a point. Let’s imagine that `divideEven`

does some blocking IO and we want to push it into another tread-pool.

```
def divideTwiceF(n: Int): Future[Option[Int]] = {
val fo = for {
n1 <- FutureO(Future(divideEven(n)))
n2 <- FutureO(Future(divideEven(n1)))
} yield n2
fo.future
}
```

And it works as expected. The `FutureO`

part there is just to alter the monadic semantics. As an exercise try to rewrite this without `FutureO`

and squirm in disgust.

### Usability

Inside for comprehension(or manual flatMaps) you can still construct failed futures, throw or return `None`

(in a future). However putting in a default value(getOrElse) is a bit trickier and going back to regular `Future`

inside same comprehension is impossible. But you can fix this. You can define methods like `orElse`

on the `FutureO`

. You can also overload the `flatMap`

to enable interop with regular futures. However this screws up type inference and I would advise against it as it could introduce some nasty bugs.

Try to implement combinators you need and leave some comments. Especially if you come across something nice or find a case where `FutureO`

is more awkward to use than regular futures.

### Theory

What we defined is actually a specialized monad transformer. Monads(such as `Future`

and `Option`

) have this nasty property that they don’t compose. You cannot write a function that takes a two monads and outputs a composed one.

However you can write such a function if you fix one of the monads and only take one as a parameter. This is called a monad transformer. In this case I fixed `Option`

. Take a closer look, we are only using `flatMap`

and a constructor(`point`

) from `Future`

. This means we could abstract over the whole monad class. And this is what Scalaz does with `OptionT`

. But to do this it needs to define a monad typeclass and instances for each and every monad they find. What I did is fix the other monad too and this produces a concrete instance you can use without any typeclasses. There is a downside of course. This is a one-off hack. If you want other transformers you’ll have to write them as well. At that point I think would be a good idea to start using Scalaz.