Scala - Playing with Records - Part Two

   
   
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This article series discuss more than 30 different programming languages. Please read overview before you read any of the details.

Playing with Records Related Articles.

Table of Content
Where to Discuss?

Local Group

Preface

Goal: Continue Part One.

Scala is powerful, but I guess my previous case example is not complex enough. We cannot just jump to this intermediate topic, with lack of basic code introduction, I need to explore scala more, and get more interesting experience. So I dedicate this article to cover basic scala trick.

4: Alternate Approach

Since our introduction to scala above is too short, consider to makes it more complete alternate flatten written in imperative fashioned.

Then we are going to continue with recursive example, and also, with head and tail x:xs pattern, until finally conclude with our songs task with alternate solution.

For Loop Using yield

We are going to use for loop. Consider start from a very simple example.

object T09f extends App {
  def loop(tags: List[String])
    = for (tag <- tags) yield tag

  val tags: List[String] =
    List("rock", "jazz", "rock", "pop", "pop")

  println(loop(tags))
}

With the result similar as below:

$ scala T09For.scala
List(rock, jazz, rock, pop, pop)

Custom Flatten Using yield

Not every real life code is functional. Some case such as concurrency are naturally comes from side effect. Thus we need to prepare to handle this, without map, filter, or reduce.

We can write our custom flatten using for loop.

import mysongs._
import mysongs.MySongs._

object T09 extends App {
  def songsFlatten(songs: List[Song])
    = for (
        song <- songs if song.tags != None;
        tag <- song.tags.get
      ) yield tag

  println(songsFlatten(songs))
}

With the result similar as below string:

$ scala T09Flatten.scala
List(60s, jazz, 60s, rock, 70s, rock, 70s, pop)

Scala: Imperative Flatten Using Yield

Exclude with Filter

Head and tail pattern matching is made of two functions:

  1. Exclude
  2. Unique

We can write the exclude function to this below:

object T10 extends App {
  def exclude(value: String, tags: List[String])
    : List[String]
    = tags filter (tag => {tag!=value})

  val tags: List[String] =
    List("rock", "jazz", "rock", "pop", "pop")

  println(exclude("rock", tags))
}

With the result similar as below list:

$ scala T10Exclude.scala
List(jazz, pop, pop)

Unique with Pattern Matching

And the unique is recursive function, with head and tail pattern matching.

object T11 extends App {
  def exclude(value: String, tags: List[String])
    : List[String]
    = tags filter (tag => {tag!=value})

  def unique(tags: List[String])
    : List[String]
     = tags match {
         case Nil    => List()
         case x::Nil => List(x)
         case x::xs  => x::unique(exclude(x, xs))
       }

  val tags: List[String] =
    List("rock", "jazz", "rock", "pop", "pop")

  println(unique(tags))
}

With the result similar as below list:

$ scala T11Unique.scala
List(rock, jazz, pop)

I guess the code itself is self explanatory.

Put All The String Together

We can apply all to finish our previous songs task, with entirely different approach.

import mysongs._
import mysongs.MySongs._

object T12 extends App {
  def songsFlatten(songs: List[Song])
    = for (
        song <- songs if song.tags != None;
        tag  <- song.tags.get
      ) yield tag

  def exclude(value: String, tags: List[String])
    : List[String]
    = tags filter (tag => {tag!=value})

  def unique(tags: List[String])
    : List[String]
     = tags match {
         case Nil    => List()
         case x::Nil => List(x)
         case x::xs  => x::unique(exclude(x, xs))
       }

  println(unique(songsFlatten(songs)))
}

With the result similar as below list:

$ scala T12Songs.scala
List(60s, jazz, rock, 70s, pop)

Scala: The Recursive of Head and Tail Pattern

We should be ready for our next topic, concurrency case example in Scala. In other time, other article.

What is Next 🤔?

Meanwhile, you can learn Lua.

Consider continue reading [ Lua - Playing with Records - Part One ].