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On my way back from Uppsala, my mind wandered to a conversation I had with a collegue about the intuition of monads, which I pretty much butchered at the time. As I was mulling this over, it dawned on me. The above is a fractal called Sierpinski triangle, the only fractal I can remember to draw. Fractals are self-similar structure like the above triangle, in which the parts are similar to the whole (in this case exactly half the scale as parent triangle).

Monads are fractals. Given a monadic data structure, its values can be composed to form another value of the data structure. This is why it’s useful to programming, and this is why it occurrs in many situations.

Let’s look at some examples:

``````scala> List(List(1), List(2, 3), List(4))
res0: List[List[Int]] = List(List(1), List(2, 3), List(4))
``````

The above is a `List` of `List` of `Int`. We can intuitively crunch this into a `List` of `Int` like this:

``````scala> List(1, 2, 3, 4)
res1: List[Int] = List(1, 2, 3, 4)
``````

For `1` to form `List(1)` we can also provide a single-parameter constructor `unit: A => F[A]`. This allows us to crunch `1` and `4` along with `List(2, 3)`:

``````scala> List(List.apply(1), List(2, 3), List.apply(4))
res2: List[List[Int]] = List(List(1), List(2, 3), List(4))
``````

The type signature of crunching, also known as `join` is `F[F[A]] => F[A]`.

### monoids

The crunching operation reminded me of monoids, which consists of:

``````trait Monoid[A] {
def mzero: A
def mappend(a1: A, a2: A): A
}
``````

We can use monoid to abstract out operations on two items:

``````scala> List(1, 2, 3, 4).foldLeft(0) { _ + _ }
res4: Int = 10

scala> List(1, 2, 3, 4).foldLeft(1) { _ * _ }
res5: Int = 24

scala> List(true, false, true, true).foldLeft(true) { _ && _ }
res6: Boolean = false

scala> List(true, false, true, true).foldLeft(false) { _ || _ }
res7: Boolean = true
``````

One aspect of monoid I want to highlight here is that data type alone is not enough to define the monoid. The pair `(Int, +)` forms a monoid. Or `Int`s are monoid under addition. See https://twitter.com/jessitron/status/438432946383360000 for more on this.

### `List` is a monad under `++`

When `List` of `List` of `Int` crunches into a `List` of `Int`, it’s obvious that it uses something like `foldLeft` and `++` to make `List[Int]`.

``````scala> List(List.apply(1), List(2, 3), List.apply(4)).foldLeft(List(): List[Int]) { _ ++ _ }
res8: List[Int] = List(1, 2, 3, 4)
``````

But it could have been something else. For example, it could return a list of sums.

``````scala> List(List.apply(1), List(2, 3), List.apply(4)).foldLeft(List(): List[Int]) { (acc, xs) => acc :+ xs.sum }
res9: List[Int] = List(1, 5, 4)
``````

That’s a contrived example, but it’s important to think of the composition semantics that a monad encapsulates.

### `Option` is a monad under…?

Let’s look at `Option` too. Remember the type signature of monadic crunching is `F[F[A]] => F[A]`, so what we need as examples are nested `Option`s, not a list of `Option`s.

``````scala> Some(None: Option[Int]): Option[Option[Int]]
res10: Option[Option[Int]] = Some(None)

scala> Some(Some(1): Option[Int]): Option[Option[Int]]
res11: Option[Option[Int]] = Some(Some(1))

scala> None: Option[Option[Int]]
res12: Option[Option[Int]] = None
``````

Here’s what I came up with to crunch `Option` of `Option` of `Int` into an `Option` of `Int`.

``````scala> (Some(None: Option[Int]): Option[Option[Int]]).foldLeft(None: Option[Int]) { (_, _)._2 }
res20: Option[Int] = None

scala> (Some(Some(1): Option[Int]): Option[Option[Int]]).foldLeft(None: Option[Int]) { (_, _)._2 }
res21: Option[Int] = Some(1)

scala> (None: Option[Option[Int]]).foldLeft(None: Option[Int]) { (_, _)._2 }
res22: Option[Int] = None
``````

So `Option` apparenlty is a monad under `_2`. In this case I don’t know if it’s immediately obvious from the implemetation, but the idea is to propagate `None`, which represents a failure.

So far we have two functions `join` and `unit`. We actually need one more, which is `map`.

• `join: F[F[A]] => F[A]`
• `unit: A => F[A]`
• `map: F[A] => (A => B) => F[B]`

Given `List[List[List[Int]]]`, we can write the accociative law by crunching the outer most list first or middle list first. The following is from one of the chapter notes of Functional Programming in Scala:

``````scala> val xs: List[List[List[Int]]] = List(List(List(1,2), List(3,4)), List(List(5,6), List(7,8)))
xs: List[List[List[Int]]] = List(List(List(1, 2), List(3, 4)), List(List(5, 6), List(7, 8)))

scala> val ys1 = xs.flatten
ys1: List[List[Int]] = List(List(1, 2), List(3, 4), List(5, 6), List(7, 8))

scala> val ys2 = xs map {_.flatten}
ys2: List[List[Int]] = List(List(1, 2, 3, 4), List(5, 6, 7, 8))

scala> ys1.flatten
res30: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8)

scala> ys2.flatten
res31: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8)
``````

This can be generalized as:

``````join(join(m)) assert_=== join(map(m)(join))
``````

Here are the identity laws also from the same notes:

``````join(unit(m)) assert_=== m
join(map(m)(unit)) assert_=== m
``````

This illustrates that we can define a monad without using `flatMap`. In actual coding, however, we tend to deal with monads by chaining `flatMap`s using `for` comprehension, which combines `map` and `join`.

### `State` monad

When writing in purely functional style, one pattern that arises often is passing a value that represents some state.

``````val (d0, _) = Tetrix.init()
val (d1, _) = Tetrix.nextBlock(d0)
val (d2, moved0) = Tetrix.moveBlock(d1, LEFT)
val (d3, moved1) =
if (moved0) Tetrix.moveBlock(d2, LEFT)
else (d2, moved0)
``````

The passing of the state object becomes boilerplate, and error-prone especially when you start to compose the state transition using function calls. `State` monad is a monad that encapsulates state transition `S => (S, A)`.

After rewriting `Tetrix.nextBlock` and `Tetrix.moveBlock` functions to return `State[GameSate, A]`, we can write the above code as:

``````def nextLL: State[GameState, Boolean] = for {
_      <- Tetrix.nextBlock
moved0 <- Tetrix.moveBlock(LEFT)
moved1 <- if (moved0) Tetrix.moveBlock(LEFT)
else State.state(moved0)
} yield moved1
nextLL.eval(Tetrix.init())
``````

It’s hard to say whether it’s good thing to be able to write `for` comprehension since it possibly makes less sense to those who are not informed about the `State` monad. One good thing is that we now have a type that automates passing `d0`, `d1`, `d2`, …

What I want to highlight here is that `State` monad is a fractal just like `List`. `moveBlock` function returns a `State` and `for` comprehension is `State` of `State`. In the above example, two calls to `moveBlock` function can be factored out:

``````def leftLeft: State[GameState, Boolean] = for {
moved0 <- Tetrix.moveBlock(LEFT)
moved1 <- if (moved0) Tetrix.moveBlock(LEFT)
else State.state(moved0)
} yield moved1
def nextLL: State[GameState, Boolean] = for {
_     <- Tetrix.nextBlock
moved <- leftLeft
} yield moved
nextLL.eval(Tetrix.init())
``````

This allows us to create mini imperative style programs that can be combined functionally. Note the semantics of `for` is limited to one monad at a time.

### `StateT` monad transformer

In the above, my hypothetical `moveBlock` returns `State[GameState, Boolean]`. When it returns `false` the block has either hit a wall or another block so no further action will be taken. If `true` do something, is like a mantra of imperative programming. It’s also a code smell for functional programming, because you likely want `Option[A]` instead. To use `State` and `Option` simultaneously, we can use `StateT`. Now all state transition will also be wrapped in `Option`.

Suppose `nextBlock` will place the current block at x position 1, and moving left beyond 0 will fail.

``````scala> import scalaz._, Scalaz._
import scalaz._
import Scalaz._

scala> :paste
// Entering paste mode (ctrl-D to finish)

type StateTOption[S, A] = StateT[Option, S, A]
object StateTOption extends StateTInstances with StateTFunctions {
def apply[S, A](f: S => Option[(S, A)]) = StateT[Option, S, A] { s =>
f(s)
}
}
case class GameState(blockPos: Int)
sealed trait Direction
case object LEFT extends Direction
case object RIGHT extends Direction
case object DOWN extends Direction
object Tetrix {
def nextBlock = StateTOption[GameState, Unit] { s =>
Some(s.copy(blockPos = 1), ())
}
def moveBlock(dir: Direction) = StateTOption[GameState, Unit] { s =>
dir match {
case LEFT  =>
if (s.blockPos == 0) None
else Some((s.copy(blockPos = s.blockPos - 1), ()))
case RIGHT => Some((s.copy(blockPos = s.blockPos + 1), ()))
case DOWN  => Some((s, ()))
}
}
}

// Exiting paste mode, now interpreting.

scala> def leftLeft: StateTOption[GameState, Unit] = for {
_ <- Tetrix.moveBlock(LEFT)
_ <- Tetrix.moveBlock(LEFT)
} yield ()
leftLeft: StateTOption[GameState,Unit]

scala> def nextLL: StateTOption[GameState, Unit] = for {
_ <- Tetrix.nextBlock
_ <- leftLeft
} yield ()
nextLL: StateTOption[GameState,Unit]

scala> nextLL.eval(GameState(0))
res0: Option[Unit] = None

scala> def nextRLL: StateTOption[GameState, Unit] = for {
_ <- Tetrix.nextBlock
_ <- Tetrix.moveBlock(RIGHT)
_ <- leftLeft
} yield ()
nextRLL: StateTOption[GameState,Unit]

scala> nextRLL.eval(GameState(0))
res1: Option[Unit] = Some(())
``````

The above shows that moving left-left failed, but calling right-left-left succeeded. In this simple example monad stacked nicely, but this could get hairly.

Another thing I was thinking on the plane was scopt, which is a command line parsing library. One of the issue that’s been raised about scopt is that the parser it generates is not composable.

If you think about it, scopt is essentially a `State`. You pass in a config case class in one end, and after series of transformations you get the config back. Here’s a hypothetical code of how scopt could look like:

``````val parser = {
val builder = scopt.OptionParser.builder[Config]("scopt")
import builder._
for {
_ <- opt[Int]('f', "foo") action { (x, c) => c.copy(foo = x) }
_ <- arg[File]("<source>") action { (x, c) => c.copy(source = x) }
_ <- arg[File]("<targets>...") unbounded() action { (x, c) => c.copy(targets = c.targets :+ x) }
} yield ()
}
parser.parse("--foo a.txt b.txt c.txt", Config()) match {
case Some(c) =>
caes None    =>
}
``````

If the `parser`’s type is `OptionParser[Unit]`, then `opt[Int]` will also be a `OptionParser[A]`. This allows us to factor out some of the options into a sub-parser and reuse it given `Config` can be reused.

### `Free` monad

Perhaps no other monads feels more fractal-like than `Free` monads. `List` and `Option` are fractal too, but with `Free` you’re involved in the construction of a nanotech monomer, which then repeats itself to become a giant structure on its own. For example, using `Tuple2[A, Next]`, `Free` can form a monad that acts like a list by embedding another `Tuple2[A, Next]` into `Next` like `Tuple2[A, Tuple2[A, Next]]`, and so on. What we end up is a data structure that’s free of additional context other than the fact that it’s a fractal. You’re responsible for destructuring the result and do something meaningful. This approach could be simpler than monad transformer.

``````scala> import scalaz._, Scalaz._
import scalaz._
import Scalaz._

scala> :paste
// Entering paste mode (ctrl-D to finish)

case class GameState(blockPos: Int)
sealed trait Direction
case object LEFT extends Direction
case object RIGHT extends Direction
case object DOWN extends Direction

sealed trait Tetrix[Next]
object Tetrix {
case class NextBlock[Next](next: Next) extends Tetrix[Next]
case class MoveBlock[Next](dir: Direction, next: Next) extends Tetrix[Next]
implicit val gameCommandFunctor: Functor[Tetrix] = new Functor[Tetrix] {
def map[A, B](fa: Tetrix[A])(f: A => B): Tetrix[B] = fa match {
case n: NextBlock[A] => NextBlock(f(n.next))
case m: MoveBlock[A] => MoveBlock(m.dir, f(m.next))
}
}
def nextBlock: Free[Tetrix, Unit] = Free.liftF[Tetrix, Unit](NextBlock(()))
def moveBlock(dir: Direction): Free[Tetrix, Unit] =
Free.liftF[Tetrix, Unit](MoveBlock(dir, ()))

def eval(s: GameState, cs: Free[Tetrix, Unit]): Option[Unit] =
cs.resume.fold({
case NextBlock(next) =>
eval(s.copy(blockPos = 1), next)
case MoveBlock(dir, next) =>
dir match {
case LEFT  =>
if (s.blockPos == 0) None
else eval(s.copy(blockPos = s.blockPos - 1), next)
case RIGHT => eval(s.copy(blockPos = s.blockPos + 1), next)
case DOWN  => eval(s, next)
}
},
{ r: Unit => Some(()) })
}

// Exiting paste mode, now interpreting.

scala> def leftLeft: Free[Tetrix, Unit] = for {
_ <- Tetrix.moveBlock(LEFT)
_ <- Tetrix.moveBlock(LEFT)
} yield ()
leftLeft: scalaz.Free[Tetrix,Unit]

scala> def nextLL: Free[Tetrix, Unit] = for {
_ <- Tetrix.nextBlock
_ <- leftLeft
} yield ()
nextLL: scalaz.Free[Tetrix,Unit]

scala> Tetrix.eval(GameState(0), nextLL)
res0: Option[Unit] = None

scala> def nextRLL: Free[Tetrix, Unit] = for {
_ <- Tetrix.nextBlock
_ <- Tetrix.moveBlock(RIGHT)
_ <- leftLeft
} yield ()
nextRLL: scalaz.Free[Tetrix,Unit]

scala> Tetrix.eval(GameState(0), nextRLL)
res1: Option[Unit] = Some(())
``````

Except for the type signature, the program portion of the code is identical to the one using `StateTOption`. There’s a bit of tradeoff on using this since we’ll be responsible for implementing the context, but there’s less mess on the type after the initial setup.

### summary

Monads are self-repeating structure like fractals, which could be expressed as a function `join: F[F[A]] => F[A]`. This property enables monadic values to be composed into larger monadic values. Just like monoid’s `mappend`, `join` can encapsulate some additional of semantics (for example `Option` and `State`). Whenever you find self-repeating structure, you might be looking at a monad. The composition of monadic types could be achieved via monad tranformers, but it is notorious for getting complicated. `Free` may offer an alternative of providing monadic DSL.