One concept that I see many times in Scalaz 7 is the monad transformer, so let’s find what that’s all about. Luckily there’s another good Haskell book that I’ve read that’s also available online.

Monad transformers 

Real World Haskell says:

It would be ideal if we could somehow take the standard State monad and add failure handling to it, without resorting to the wholesale construction of custom monads by hand. The standard monads in the mtl library don’t allow us to combine them. Instead, the library provides a set of monad transformers to achieve the same result.

A monad transformer is similar to a regular monad, but it’s not a standalone entity: instead, it modifies the behaviour of an underlying monad.

Reader, yet again 

Let’s translate the Reader monad example into Scala:

scala> def myName(step: String): Reader[String, String] = Reader {step + ", I am " + _}
myName: (step: String)scalaz.Reader[String,String]

scala> def localExample: Reader[String, (String, String, String)] = for {
         a <- myName("First")
         b <- myName("Second") >=> Reader { _ + "dy"}
         c <- myName("Third")  
       } yield (a, b, c)
localExample: scalaz.Reader[String,(String, String, String)]

scala> localExample("Fred")
res0: (String, String, String) = (First, I am Fred,Second, I am Freddy,Third, I am Fred)

The point of Reader monad is to pass in the configuration information once and everyone uses it without explicitly passing it around. See Configuration Without the Bugs and Gymnastics by Tony Morris (@dibblego).

ReaderT 

Here’s an example of stacking ReaderT, monad transformer version of Reader on Option monad.

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

type ReaderTOption[A, B] = ReaderT[Option, A, B]
object ReaderTOption extends KleisliInstances with KleisliFunctions {
  def apply[A, B](f: A => Option[B]): ReaderTOption[A, B] = kleisli(f)
}

// Exiting paste mode, now interpreting.

Now using ReaderTOption object, we can create a ReaderTOption:

scala> def configure(key: String) = ReaderTOption[Map[String, String], String] {_.get(key)}
configure: (key: String)ReaderTOption[Map[String,String],String]

On day 2 we mentioned about considering Function1 as an infinite map. Here we are doing sort of the opposite by using Map[String, String] as a reader.

scala> def setupConnection = for {
         host <- configure("host")
         user <- configure("user")
         password <- configure("password")
       } yield (host, user, password)
setupConnection: scalaz.Kleisli[Option,Map[String,String],(String, String, String)]

scala> val goodConfig = Map(
         "host" -> "eed3si9n.com",
         "user" -> "sa",
         "password" -> "****"
       )
goodConfig: scala.collection.immutable.Map[String,String] = Map(host -> eed3si9n.com, user -> sa, password -> ****)

scala> setupConnection(goodConfig)
res2: Option[(String, String, String)] = Some((eed3si9n.com,sa,****))

scala> val badConfig = Map(
         "host" -> "example.com",
         "user" -> "sa"
       )
badConfig: scala.collection.immutable.Map[String,String] = Map(host -> example.com, user -> sa)

scala> setupConnection(badConfig)
res3: Option[(String, String, String)] = None

As you can see the above ReaderTOption monad combines Reader’s ability to read from some configuration once, and Option’s ability to express failure.

Stacking multiple monad transformers 

RWH:

When we stack a monad transformer on a normal monad, the result is another monad. This suggests the possibility that we can again stack a monad transformer on top of our combined monad, to give a new monad, and in fact this is a common thing to do.

We can stack StateT to represent state transfer on top of ReaderTOption.

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

type StateTReaderTOption[C, S, A] = StateT[({type l[X] = ReaderTOption[C, X]})#l, S, A]

object StateTReaderTOption extends StateTInstances with StateTFunctions {
  def apply[C, S, A](f: S => (S, A)) = new StateT[({type l[X] = ReaderTOption[C, X]})#l, S, A] {
    def apply(s: S) = f(s).point[({type l[X] = ReaderTOption[C, X]})#l]
  }
  def get[C, S]: StateTReaderTOption[C, S, S] =
    StateTReaderTOption { s => (s, s) }
  def put[C, S](s: S): StateTReaderTOption[C, S, Unit] =
    StateTReaderTOption { _ => (s, ()) }
}

// Exiting paste mode, now interpreting.

This is a bit confusing. Ultimately the point of State monad is to wrap S => (S, A), so I kept those parameter names. Next, we need to modify the kind of ReaderTOption to * -> * (a type constructor that takes exactly one type as its parameter).

Suppose we want to implement Stack using state like we did in day 7.

scala> type Stack = List[Int]
defined type alias Stack

scala> type Config = Map[String, String]
defined type alias Config

scala> val pop = StateTReaderTOption[Config, Stack, Int] {
         case x :: xs => (xs, x)
       }
pop: scalaz.StateT[[+X]scalaz.Kleisli[Option,Config,X],Stack,Int] = StateTReaderTOption$$anon$1@122313eb

Since I wrote get and put we should be able to write it using for syntax as well:

scala> val pop: StateTReaderTOption[Config, Stack, Int] = {
         import StateTReaderTOption.{get, put}
         for {
           s <- get[Config, Stack]
           val (x :: xs) = s
           _ <- put(xs)
         } yield x
       }
pop: StateTReaderTOption[Config,Stack,Int] = scalaz.StateT$$anon$7@7eb316d2

Here’s push:

scala> def push(x: Int): StateTReaderTOption[Config, Stack, Unit] = {
         import StateTReaderTOption.{get, put}
         for {
           xs <- get[Config, Stack]
           r <- put(x :: xs)
         } yield r
       }
push: (x: Int)StateTReaderTOption[Config,Stack,Unit]

We can also port stackManip:

scala> def stackManip: StateTReaderTOption[Config, Stack, Int] = for {
         _ <- push(3)
         a <- pop
         b <- pop
       } yield(b)
stackManip: StateTReaderTOption[Config,Stack,Int]

Here’s how we run this.

scala> stackManip(List(5, 8, 2, 1))(Map())
res12: Option[(Stack, Int)] = Some((List(8, 2, 1),5))

So far we have the same feature as the State version. Let’s modify configure:

scala> def configure[S](key: String) = new StateTReaderTOption[Config, S, String] {
         def apply(s: S) = ReaderTOption[Config, (S, String)] { config: Config => config.get(key) map {(s, _)} }
       }
configure: [S](key: String)StateTReaderTOption[Config,S,String]

Using this we can now manipulate the stack using read-only configuration:

scala> def stackManip: StateTReaderTOption[Config, Stack, Unit] = for {
         x <- configure("x")
         a <- push(x.toInt)
       } yield(a)

scala> stackManip(List(5, 8, 2, 1))(Map("x" -> "7"))
res21: Option[(Stack, Unit)] = Some((List(7, 5, 8, 2, 1),()))

scala> stackManip(List(5, 8, 2, 1))(Map("y" -> "7"))
res22: Option[(Stack, Unit)] = None

Now we have StateT, ReaderT and Option working all at the same time. Maybe I am not doing it right, but setting this up defining StateTReaderTOption and configure was painful. The usage code (stackManip) looks clean so we might do these things for special occasions like Thanksgiving.

It was rough without LYAHFGG, but we will pick it up from here later.