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About

This repo is a basis of a guest lecture held at NTNU Trondheim in the course TDT4165 Programming Languages.

The goal is to show of as much as possible of Scala's base features in 2 hours and give motivations for why thinking of types and domains is useful and important.

Scala at FINN.no

Finn has been deploying scala for at least 8 years now. Most of the teams using it is embracing the more functional style. Some things we really like:

  • Errors at compile time instead of at run time
  • Immutability first - approach
  • Incredibly powerful type system
  • Concise and regular language structure
  • Very nice library support for doing Functional Programming
  • Leverage existing tools from JVM eco system
  • Ability to drop down to procedural when desired

Mini-FINN

A barebone classified ads service using the console

Setup for Intellij

  • Install the scala plugin (Preferences -> Plugins -> Search 'scala' -> Restart Intellij)
  • Open project in Intellij
  • Choose project JDK 1.8 (only tested on java8), leave rest as defaults

Running

  • Press the play button on a main class (AdServer in a sub-module)

Prelude

Read: Syntax

Task 0: Introduction

  • We have a small service for inputting and outputting Ads.
  • It uses the console to select mode (input, output, exit).
  • We "mock" a database using a mutable Map

Task 1: ADT

Read: ADTExample

Problem: The modus variable in AdServer is implemented as a string and such has an infinite domain

Task: Replace it with an ADT Sum Type with a limited domain.

Task 2: Modeling

Read: ModelingExample

Problem: The database accepts strings and not product types

Task: Implement an ADT for two kinds of Ad

  • CarAd, with fields regNr (registration number) and price
  • JobAd, with fields company and monthly salary

Make sure we can output Ads to the console in the same format as it is input

Task 3: Option

Read: OptionExample

Problem: UnknownAdType is not meaningful in the Ad type

Task: Get rid of UnknownAdType. Do this by using the Option type

Task 4: Either

Read: EitherExample

Problem: The user gets no feedback on what went wrong if we could not handle the input

Task: Switch previous tasks use of Options in favour of Either with a custom Error-ADT

Task 5: Recursion

Read: RecursionExample

Problem: The app uses a while loop mutating a var. Eg it has state.

Task: Remove state in AdServer by making the run method tail-recursive

Task 6: IO

Read: IOExample

Problem: There are untracked side effects in accessing console and database

Task: Wrap up console effects in IO and compose program as IO.

Bonus: wrap up all "Database"-read/write in IOs to)

Bonus2: create list of modes to present user