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Process text file concurrently

 Process the same input file concurrently Let's say we have a text file that contains 2 different types of lines.  For the sake of example, one group starts with "i:" (for Integer) and the other group starts with "s:" (for String), something like the following file: s:New york s:Apple i:387548 s:Amsterdam i:4556 i:39874 s:Orange i:56787 s:Banana i:4657567 s:Turkey i:45679456 s:Iran i:4356456 i:23423 i:456 s:Ukraine i:453645 i:5456 We want to process these input lines separately but concurrently. And the process can be anything, for this example, we just print them to the console. But what matters is that we open the file only for READ so 2 different processes (or routines) can read the file at the same time. The Cats Effect answer to open files (or in general any resources) in a safe and efficient way, is  Resource class. So we need a function to take the path to the file and give us a Resource of some type that allows us to read the file line by line. One o...
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Using IO class to perform side-effects in a pure functional way

 Cats-effects' IO class is the answer of the library to performing side effects (or simply effects) in a functional way. Functional programming is focused on pure functions and a pure function is a function that does not perform any side effects but only produces an output based on the receiving inputs. Real-world programs are far from this mathematical abstract definition. A program with no side effects is doing nothing as most of the useful things are side effects, such as writing to a database, writing to a file, reading from a file, printing on the screen, and ...



An IO class is actually a description of a computation, rather than the execution of that computation. That's why it will not make a function an impure function.

Example:

Printing on the screen (which is a side effect), as some sort of logging.

If we want to print on the screen using inside the body of a function the side effect happens but if we return the effect as an IO object wrapping the calculation result, then we have a pure function that delegates the effects to its caller.


Bad function (not pure function):

  def notPureMultiple(a: Int, b: Int): Int = {
    // some kind of logging inside the function
    println(s"not pure: calculating $a * $b")
    a * b
  }

Good function (pure function):
  def pureMultiple(a: Int, b: Int): IO[Int] = {
    IO {
      println(s"pure: calculating $a * $b")
      a * b
    }
  }


Complete code:

package com.blogspot.scaland7.catseffects

import cats.effect.IO
import cats.effect.unsafe.implicits.global

object SideEffects01 {

  def notPureMultiple(a: Int, b: Int): Int = {
    // some kind of logging inside the function
    println(s"not pure: calculating $a * $b")
    a * b
  }

  def pureMultiple(a: Int, b: Int): IO[Int] = {
    IO {
      println(s"pure: calculating $a * $b")
      a * b
    }
  }

  def main(args: Array[String]): Unit = {
    // this calculates the result and do the side effect right away, which is not good!
    val value1: Int = notPureMultiple(10, 20)

    // this return an IO of the result so the side effect has not happened until we run the io
    val value2: IO[Int] = pureMultiple(20, 10)
    value2.unsafeRunSync() // effects happens here
  }
}

For cats-effects dependency add the following line to your build.sbt file

libraryDependencies += "org.typelevel" %% "cats-effect" % "3.4.8"

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