Imagine that you are writing function of calculating the Fibonacci series like this and add a print statement for debugging purposes:

def fib(n:Int) : Int = {
  if(n == 0 || n ==1) {
    println(s"base case : $n")
    n
  }
  else {
    println(s"add fib(n-1) + fib(n-2) $n")
    fib(n-1) + fib(n-2)
  }
}

Then, you run the following function:

fib(5)

// interpreter
add fib(n-1) + fib(n-2) 5
add fib(n-1) + fib(n-2) 4
add fib(n-1) + fib(n-2) 3
add fib(n-1) + fib(n-2) 2
base case : 1
base case : 0
base case : 1
add fib(n-1) + fib(n-2) 2
base case : 1
base case : 0
add fib(n-1) + fib(n-2) 3
add fib(n-1) + fib(n-2) 2
base case : 1
base case : 0
base case : 1
fib: (n: Int)Int
res0: Int = 5

A condition of executing multiple functions synchronously, it works just fine:

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

fib(5)
fib(4)
fib(3)

// Exiting paste mode, now interpreting.

add fib(n-1) + fib(n-2) 5
add fib(n-1) + fib(n-2) 4
add fib(n-1) + fib(n-2) 3
add fib(n-1) + fib(n-2) 2
base case : 1
base case : 0
base case : 1
add fib(n-1) + fib(n-2) 2
base case : 1
base case : 0
add fib(n-1) + fib(n-2) 3
add fib(n-1) + fib(n-2) 2
base case : 1
base case : 0
base case : 1
add fib(n-1) + fib(n-2) 4
add fib(n-1) + fib(n-2) 3
add fib(n-1) + fib(n-2) 2
base case : 1
base case : 0
base case : 1
add fib(n-1) + fib(n-2) 2
base case : 1
base case : 0
add fib(n-1) + fib(n-2) 3
add fib(n-1) + fib(n-2) 2
base case : 1
base case : 0
base case : 1
res2: Int = 2

However, if the function is wrapped in an asynchronous manner, it will be really hard to tell which log is associated to which. In this scenario, how can we debug an operation that is in asynchronous manner?

Writer Data Types to the Rescue

Cats have a writer data type class that can help you rescue by attaching your log statement with the underlying result value so that you can understand log statements asynchronously.

In Cats, Writer data types definition is: Writer[L, V].

L is the logging type collection Monoid that you want to have; in this case, we can set L as a Vector[String].

V is the result type of the function operation - in this case, we can set V as Int since we are returning an integer type from the Fibonacci.

Initialization

Once you create know what is L and V in the definition, you can create your Writer Data Type like this:

import cats.data.Writer
import cats.implicits._

Writer(Vector("log1", "log2"), 0)

or

import cats.data.Writer
import cats.implicits._

0.writer(Vector("log1", "log2"))

If you saw the repl, or the result, you will realize that it is not Writer[L, V], but return a WrtierT[Id, L, V]. It is because cats use type alias to derive the value of Writer from WriterT. In this post, we are going to talk about how to use Writer. Therefore, you can ignore the details and treat the type WriterT[Id, L, V] as Writer[L, V].

Having log value but no result

If there is a log but no result we can use tell:

Vector("msg1", "msg2").tell()

We can also extract both the output and the logs at the same time with run:

val writer = Writer(Vector("something"), 0)
val (log, result) = writer.run

Extract Result value and Log type

Extract the result and log with value and written respectively:

val a = Writer(Vector("msg1"),0)
val log = a.written
val result = a.value

println(s"log: $log result: $result")
// log: Vector(msg1) result: 0

Composing and Transforming Writers

Since Writer is a Monad, you can do operation on Writer with map and flatmap.

flatMap combines the log type and also the result type together from the source Writer and the result of the sequencing function.

Therefore, it is a good practice to put a Log type that has an efficient of append and concatenate method, such as Vector:

val res = for {
    a <- Writer(Vector("a"), 1)
    _ <- Vector("c").tell
    b <- 3.writer(Vector("3", "b"))
  } yield {
    println(s"a $a") // 1
    println(s"b $b") // 3
    a + b // 4
  }

println(res) //WriterT((Vector(a, c, 3, b),4))

Note that the tell method will preserve the original Writer and append the “c” to the source Writer, which is “a”.

The result of the Writer is based on what will be computed after the yield function. If there is no addition after yield, and only a, for instance, the end result will be WriterT((Vector(a,c,3,b),1)).

Transforming Writer

We can change the Log type to all upperCase by using mapWritten:

// .. take example from previous res example
val upperCaseLog = res.mapWritten(previousLog => previousLog.map(_.toUpperCase))
  println(upperCaseLog) 
  // WriterT((Vector(A, C, 3, B),4))

You can also transform both type by using mapBoth:

val newWriterValueAndLog = res.mapBoth{ (log,res) =>
    (log :+ "appending z", res+12)
  }
println(newWriterValueAndLog)

WriterT((Vector(a, c, 3, b, appending z),16))

Swap the log type and the result using swap:

val swappedWriter = res.swap
println(swappedWriter)
// WriterT((4,Vector(a, c, 3, b)))

Last but not least, reset the log value in Writer using reset:

val resetWriter = res.reset
println(resetWriter)

// WriterT((Vector(),4))

Writer in Action

Now you have read through this far and know what a Writer is, let’s refactor our code to incorporate Writer in it:

First, let’s add a timeOut function to set up the asynchronous environment.

 def timeout[A](body: => A):A = try {
    body
  } finally Thread.sleep(100)

Then we set a Type alias of LogFib from Writer:

type LogFib[A] = Writer[Vector[String], A]

We change the Fib function to return a LogFib[Int]:

def fib(n:Int): LogFib[Int] = {
    timeout(
      if(n == 0 || n ==1) {
        n.writer(Vector(s"base case : $n"))
      }
      else {
        for {
          _ <- Vector(s"add fib(n-1) + fib(n-2) $n").tell
          fib1 <- fib(n-1)
          fib2 <- fib(n-2)
        } yield fib1 + fib2
      }
    )
  }

Then you can run it like this :

import scala.concurrent.duration._
  implicit val ec :ExecutionContext = scala.concurrent.ExecutionContext.Implicits.global
  val fibRes = Await.result(Future.sequence(Vector(
    Future(fib(5)),
    Future(fib(4)),
    Future(fib(3))
  ))
  , Duration.Inf)


  fibRes.toList.map(w => {
    val (logging, endResult) = w.run
    println(s"logging $logging endResult $endResult")
  })

Takeaway

• Writer Data Type is useful for a logging operation in a multi-threaded environment
• The Writer Log is tied to the result. Therefore, it is an excellent way to record the sequence of multi-threaded computation.

All the example information are in github

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