Scala Practice

Optional language. Scala is not one of the languages this curriculum targets, so these exercises have no reference implementations yet and are excluded from ol verify build and from CI. They are kept because the exercises are worth doing, and every ol command works on them by name.

Toolchain: brew install scala-cli. On a chezmoi-managed machine it comes from the install_alt_langs flag, which is off by default.

scala-cli runs a bare directory of sources and manages its own JVM, so no build definition is needed.

Master functional programming, the type system, and pattern matching on the JVM.

Language Essentials

Concept	Details
Memory Model	JVM GC, value classes, case classes (immutable by default)
Type System	Strong static, higher-kinded types, variance, path-dependent types
Error Handling	Option, Either, Try, Future recovery, no checked exceptions
Concurrency	Future/Promise, Akka actors, cats-effect IO, ZIO
Build	sbt, scala-cli, Mill
Standard	Scala 3 (Dotty)

Key Idioms

• Case classes for immutable data
• Pattern matching with exhaustiveness checking
• For-comprehensions for monadic composition
• Implicits / given instances (Scala 3) for type-class derivation
• Sealed traits for algebraic data types
• Extension methods (Scala 3) over implicit classes
• Opaque types for zero-cost newtype wrappers

Reference Documentation

• Scala 3 Book — free, official
• Scala Standard Library — API docs
• Functional Programming in Scala (Chiusano & Bjarnason) — the red book
• Scala Exercises — free interactive
• Typelevel ecosystem — cats, fs2, http4s

Practising these

practice/bin/ol list  build scala          # which of these have references yet
practice/bin/ol start build scala 01       # clone it stubbed into .scratchpad/
practice/bin/ol check build scala 01       # run it; exit code is the verdict
practice/bin/ol diff  build scala 01       # your attempt against the reference

Exercises are ordered by concept, not by difficulty, so pick by the stars rather than by the number. An exercise listed below with no reference yet is a row in this table and nothing more; see the practice path for how the harness works.

Exercises

#	Exercise	Concepts	Difficulty
01	Immutable red-black tree	Case classes, pattern matching, recursion	⭐⭐⭐
02	Monad implementation	Higher-kinded types, flatMap, for-comprehension	⭐⭐⭐⭐
03	Actor system (simplified)	Message passing, mailboxes, supervision	⭐⭐⭐⭐
04	Type-safe builder (phantom types)	Phantom types, type-level state machine	⭐⭐⭐⭐
05	Streaming CSV parser	Iterator, lazy evaluation, fs2-style	⭐⭐⭐
06	JSON codec (automatic derivation)	Given instances, Mirror, compile-time derivation	⭐⭐⭐⭐⭐
07	Parallel collection processor	Future, ExecutionContext, Par	⭐⭐⭐
08	Free monad interpreter	GADTs, natural transformations, Church encoding	⭐⭐⭐⭐⭐
09	Property-based test framework	Generators, shrinking, given Arbitrary	⭐⭐⭐⭐
10	Effect system (mini-ZIO)	Trampolining, fibers, error channels	⭐⭐⭐⭐⭐