Iterator[String]

An Iterator[String] is what you get after invoking scala.io.Source.fromFile(file).getLines (see foreach school). When you import implicits from com.ambiata.origami.FoldableM you can turn this Iterator into a FoldableM and apply FoldMs to it.

State folding

Let's go back to the example of counting the number of lines and totalLineSize of a file described in foreach school:

import com.ambiata.origami._, Origami._
import scalaz._, Scalaz._

val source = scala.io.Source.fromFile("file.txt")

val totalSize: Fold[String, Int] =
  fromMonoidMap[String, Int](_.size)

val result: (Int, Int) =
  (count <*> totalSize).run(source)

(see implicits for a description of imports)

This solves the "composition" problem because it is now very easy to develop and test a new piece of folding, totalSize, and add it to the current processing, count.

This is however not very clean because:

• the fold is executed right away, as a side-effect

• there is no resource management

The IO monad

We need, at least, to handle side effects. In order to do this we can execute our fold in the IO monad:

import com.ambiata.origami._, Origami._
import scalaz._, Scalaz._
import scalaz.effect._

val source = scala.io.Source.fromFile("file.txt")

val totalSize: Fold[String, Int] =
  fromMonoidMap[String, Int](_.size)

val result: IO[(Int, Int)] =
  (count <*> totalSize).into[IO].run(source)

This is better, now we need to close the source when we are done with it.

SafeTIO

import com.ambiata.origami._, Origami._
import scalaz._, Scalaz._

val source = scala.io.Source.fromFile("file.txt")


val totalSize: Fold[String, Int] =
  fromMonoidMap[String, Int](_.size)

val result: SafeTIO[(Int, Int)] =
  (count <*> totalSize).into[SafeTIO].run(source)

SafeTIO is a type alias for SafeT[IO, ?] (see Type aliases for more definitions) and is just a small layer on top of IO to register "finalizers" which will execute when an IO action executes whether it succeeds or not (see SafeT).

The result is a SafeTIO[(Int, Int)] which we can run to get a proper IO value:

 val ioResult: IO[(Int, Int)] =
   result.run