_________             .__
 /   _____/ ____ _____  |  | _____
 \_____  \_/ ___\\__  \ |  | \__  \
 /        \  \___ / __ \|  |__/ __ \_
/_______  /\___  >____  /____(____  /
        \/     \/     \/          \/
___________
\__    ___/__.__.______   ____
  |    | <   |  |\____ \_/ __ \
  |    |  \___  ||  |_> >  ___/
  |____|  / ____||   __/ \___  >
          \/     |__|        \/
_________ .__
\_   ___ \|  | _____    ______ ______ ____   ______
/    \  \/|  | \__  \  /  ___//  ___// __ \ /  ___/
\     \___|  |__/ __ \_\___ \ \___ \\  ___/ \___ \
 \______  /____(____  /____  >____  >\___  >____  >
        \/          \/     \/     \/     \/     \/
___________                             .__
\_   _____/__  ________    _____ ______ |  |   ____
 |    __)_\  \/  /\__  \  /     \\____ \|  | _/ __ \
 |        \>    <  / __ \|  Y Y  \  |_> >  |_\  ___/
/_______  /__/\_ \(____  /__|_|  /   __/|____/\___  >
        \/      \/     \/      \/|__|             \/

Type classes example in Scala

by dgk42

Requirements

• JDK 1.7 or higher

• sbt

  For instructions on installing sbt on your system, see this.

Contents

Goal: We want to write a class to express approximate equality. We don't want the example to be dead trivial, so we used an auxiliary trait that expresses the distance property for a type. Please bear in mind that this is just an example - it's easy to understand that a pragmatic implementation of approximate equality could deviate from this example.

How-to: We implement the above in 2 different ways:

a) using parametric polymorphism. For implementation details, see the ApproxG.scala file

b) using ad-hoc polymorphism with type classes. For implementation details, see the ApproxTC.scala file

Rule of thumb: The G in a class name implies (a), while the TC implies (b).

Property testing using ScalaCheck is also implemented for both (a) and (b).

Note 1: We deliberately import the Scalaz framework, since most of the time one shall need stuff from there when working with these.

Note 2: To see important comments, issue the following:

grep -nr NOTE src

in the project's root directory.

Building and running

sbt compile test

sbt "runMain dgk.approxequal.GExample"

sbt "runMain dgk.approxequal.TCExample"

• ScalaIDE

  To create an Eclipse project ready to be imported in the IDE, fire sbt eclipse

• IntelliJ

  Use the IDEA sbt plug-in to load the project

Presentation

There's also a .pdf located in the root directory covering a presentation about the topic.

Exercises for the reader

• TODO tasks

  grep -nr TODO src

• Remove the WithDistance trait and provide a simpler implementation

  See how many implicits you can prune!

• ScalaCheck reports that the transitive property holds. Try to tweak it, so it can find a counter-example

  In the example apps (GExample and TCExample) you can find more clues.

• Try to implement a type class of your own.

Final words

• We love FP!

• We love Haskell!

• We love Scalaz!