Scala Cheat Sheet

Learn Scala from the basics →

Values & Types

val name = "Ada": Immutable value (prefer by default)
var count = 0: Mutable variable
val price: Double = 9.99: Explicit type annotation
lazy val config = load(): Computed on first use
"42".toInt / "42".toIntOption: Convert — throwing vs Option-returning

Strings

s"Hello, $name! Next year: ${age + 1}": String interpolation
f"$price%.2f": Formatted interpolation (printf style)
raw"C:\temp\new": No escape processing
"""|line 1 ... |line 2""".stripMargin: Multi-line string; stripMargin removes everything up to each |
s.length / s.toUpperCase / s.trim / s.split(","): Common string methods

Control Flow

val kind = if n > 0 then "positive" else "not positive": if is an expression
x match { case 0 => "zero"; case n if n < 0 => "negative"; case _ => "positive" }: Pattern matching with a guard
for i <- 1 to 5 do println(i): Loop over a range (1 until 5 excludes 5)
for x <- xs if x > 0 yield x * 2: for-comprehension producing a collection
while i < 10 do i += 1: while loop

Methods & Functions

def area(w: Double, h: Double): Double = w * h: Method
def greet(name: String = "world") = s"Hi, $name": Default parameter
greet(name = "Ada"): Named argument
val double = (x: Int) => x * 2: Function value (lambda)
xs.map(_ * 2): Placeholder syntax for short lambdas

Collections

List(1, 2, 3) / Vector(1, 2, 3): Immutable sequences
Set("a", "b") / Map("a" -> 1, "b" -> 2): Immutable set and map
xs.filter(_ > 1).map(_ * 10).sum: Transform pipeline
xs.foldLeft(0)(_ + _): Fold to a single value
people.groupBy(_.city) / xs.sortBy(-_): Group and sort
m.getOrElse("c", 0) / m + ("c" -> 3): Map lookup with default / add

Classes & Types

case class Point(x: Int, y: Int): Immutable data class with equality and copy
p.copy(y = 5): Copy with changes
trait Shape { def area: Double }: Trait (interface with optional implementation)
enum Color { case Red, Green, Blue }: Enumeration
object Config { val port = 8080 }: Singleton object
extension (s: String) def shout = s.toUpperCase + "!": Extension method

Handling Absence & Errors

Option(x) / Some(1) / None: Optional value instead of null
opt.map(_ + 1).getOrElse(0): Transform or use a default
Try("x".toInt): Success(value) or Failure(exception)
Either[String, Int] / Right(1) / Left("error"): Result with a typed error
try risky() catch { case e: NumberFormatException => 0 }: try/catch with pattern matching

Tools

scala run hello.scala: Compile and run a file (Scala CLI)
scala repl: Interactive REPL
//> using dep com.lihaoyi::upickle:4.1.0: Add a library in a Scala CLI script
sbt compile / sbt test / sbt run: Build, test, and run an sbt project
scala test .: Run tests with Scala CLI