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

• ~20 years old
• Attempts to fuse object oriented and functional programing
• Runs on JVM
• Has pioneered many of the features we see spreading the last years.
• Version 3 released "recently". Embraces braceless syntax and is compatible with 2.

Scala at FINN.no

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
• 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

Setup for VsCode (and others)

• Install the plugin "Metals"

Running

• Press the play button in Main.scala

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
• The id of ads is easily mistaken for other ids
• Our ads are just strings, make them have both a price and some text.

Task:

• Replace Modus with an Sum Type with a limited domain.
• Introduce a opaque type for AdId
• Introduce a Product Type for Ad

Task 2: Modeling

Read: ModelingExample

Problem: The database accepts strings and not product types

Task: Implement an ADT for two kinds of Ad and parse them from string

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

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

Task 3: Option

Read: OptionExample

Problem: UnknownAdType and Unknown mode are easily mistaken for good results

Task: Get rid of Unknown types. Also upgrade the database to be able to signal missing element.

Task 4: Either / disjuctions

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 for unknowns in favor of error-signaling types

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 and database effects in IO and compose program as IO.

Bonus 7: State

Read: RefExample

Problem: The database is concrete for each server. We want to be able to safely use it concurrently with N servers/users.

Task: Wrap up the mutability in Database and pass a database instance into server on startup.