Scala Cheatsheet

Scala Cheatsheet

Language

Thanks to Brendan O’Connor, this cheatsheet aims to be a quick reference of Scala syntactic constructions. Licensed by Brendan O’Connor under a CC-BY-SA 3.0 license.

variables
var x = 5

Good
x = 6
Variable.
val x = 5

Bad
x = 6
Constant.
var x: Double = 5
Explicit type.
functions
Good
def f(x: Int) = { x * x }

Bad
def f(x: Int)   { x * x }
Define function.
Hidden error: without = it’s a procedure returning Unit; causes havoc. Deprecated in Scala 2.13.
Good
def f(x: Any) = println(x)

Bad
def f(x) = println(x)
Define function.
Syntax error: need types for every arg.
type R = Double
Type alias.
def f(x: R)
vs.
def f(x: => R)
Call-by-value.

Call-by-name (lazy parameters).
(x: R) => x * x
Anonymous function.
(1 to 5).map(_ * 2)
vs.
(1 to 5).reduceLeft(_ + _)
Anonymous function: underscore is positionally matched arg.
(1 to 5).map(x => x * x)
Anonymous function: to use an arg twice, have to name it.
(1 to 5).map { x =>
  val y = x * 2
  println(y)
  y
}
Anonymous function: block style returns last expression.
(1 to 5) filter {
  _ % 2 == 0
} map {
  _ * 2
}
Anonymous functions: pipeline style (or parens too).
def compose(g: R => R, h: R => R) =
  (x: R) => g(h(x))

val f = compose(_ * 2, _ - 1)
Anonymous functions: to pass in multiple blocks, need outer parens.
val zscore =
  (mean: R, sd: R) =>
    (x: R) =>
      (x - mean) / sd
Currying, obvious syntax.
def zscore(mean: R, sd: R) =
  (x: R) =>
    (x - mean) / sd
Currying, obvious syntax.
def zscore(mean: R, sd: R)(x: R) =
  (x - mean) / sd
Currying, sugar syntax. But then:
val normer =
  zscore(7, 0.4) _
Need trailing underscore to get the partial, only for the sugar version.
def mapmake[T](g: T => T)(seq: List[T]) =
  seq.map(g)
Generic type.
5.+(3); 5 + 3

(1 to 5) map (_ * 2)
Infix sugar.
def sum(args: Int*) =
  args.reduceLeft(_+_)
Varargs.
packages
import scala.collection._
Wildcard import.
import scala.collection.Vector

import scala.collection.{Vector, Sequence}
Selective import.
import scala.collection.{Vector => Vec28}
Renaming import.
import java.util.{Date => _, _}
Import all from java.util except Date.
At start of file:
package pkg

Packaging by scope:
package pkg {
  ...
}

Package singleton:
package object pkg {
  ...
}
Declare a package.
data structures
(1, 2, 3)
Tuple literal (Tuple3).
var (x, y, z) = (1, 2, 3)
Destructuring bind: tuple unpacking via pattern matching.
Bad
var x, y, z = (1, 2, 3)
Hidden error: each assigned to the entire tuple.
var xs = List(1, 2, 3)
List (immutable).
xs(2)
Paren indexing (slides).
1 :: List(2, 3)
Cons.
1 to 5
same as
1 until 6

1 to 10 by 2
Range sugar.
()
Empty parens is singleton value of the Unit type.
Equivalent to void in C and Java.
control constructs
if (check) happy else sad
Conditional.
if (check) happy

same as
if (check) happy else ()
Conditional sugar.
while (x < 5) {
  println(x)
  x += 1
}
While loop.
do {
  println(x)
  x += 1
} while (x < 5)
Do-while loop.
import scala.util.control.Breaks._

breakable {
  for (x <- xs) {
    if (Math.random < 0.1)
      break
  }
}
Break (slides).
for (x <- xs if x % 2 == 0)
  yield x * 10

same as
xs.filter(_ % 2 == 0).map(_ * 10)
For-comprehension: filter/map.
for ((x, y) <- xs zip ys)
  yield x * y

same as
(xs zip ys) map {
  case (x, y) => x * y
}
For-comprehension: destructuring bind.
for (x <- xs; y <- ys)
  yield x * y

same as
xs flatMap { x =>
  ys map { y =>
    x * y
  }
}
For-comprehension: cross product.
for (x <- xs; y <- ys) {
  val div = x / y.toFloat
  println("%d/%d = %.1f".format(x, y, div))
}
For-comprehension: imperative-ish.
sprintf style.
for (i <- 1 to 5) {
  println(i)
}
For-comprehension: iterate including the upper bound.
for (i <- 1 until 5) {
  println(i)
}
For-comprehension: iterate omitting the upper bound.
pattern matching
Good
(xs zip ys) map {
  case (x, y) => x * y
}

Bad
(xs zip ys) map {
  (x, y) => x * y
}
Use case in function args for pattern matching.
Bad
val v42 = 42
3 match {
  case v42 => println("42")
  case _   => println("Not 42")
}
v42 is interpreted as a name matching any Int value, and “42” is printed.
Good
val v42 = 42
3 match {
  case `v42` => println("42")
  case _     => println("Not 42")
}
`v42` with backticks is interpreted as the existing val v42, and “Not 42” is printed.
Good
val UppercaseVal = 42
3 match {
  case UppercaseVal => println("42")
  case _            => println("Not 42")
}
UppercaseVal is treated as an existing val, rather than a new pattern variable, because it starts with an uppercase letter. Thus, the value contained within UppercaseVal is checked against 3, and “Not 42” is printed.
object orientation
class C(x: R)
Constructor params - x is only available in class body.
class C(val x: R)

var c = new C(4)

c.x
Constructor params - automatic public member defined.
class C(var x: R) {
  assert(x > 0, "positive please")
  var y = x
  val readonly = 5
  private var secret = 1
  def this = this(42)
}
Constructor is class body.
Declare a public member.
Declare a gettable but not settable member.
Declare a private member.
Alternative constructor.
new {
  ...
}
Anonymous class.
abstract class D { ... }
Define an abstract class (non-createable).
class C extends D { ... }
Define an inherited class.
class D(var x: R)

class C(x: R) extends D(x)
Inheritance and constructor params (wishlist: automatically pass-up params by default).
object O extends D { ... }
Define a singleton (module-like).
trait T { ... }

class C extends T { ... }

class C extends D with T { ... }
Traits.
Interfaces-with-implementation. No constructor params. mixin-able.
trait T1; trait T2

class C extends T1 with T2

class C extends D with T1 with T2
Multiple traits.
class C extends D { override def f = ...}
Must declare method overrides.
new java.io.File("f")
Create object.
Bad
new List[Int]

Good
List(1, 2, 3)
Type error: abstract type.
Instead, convention: callable factory shadowing the type.
classOf[String]
Class literal.
x.isInstanceOf[String]
Type check (runtime).
x.asInstanceOf[String]
Type cast (runtime).
x: String
Ascription (compile time).
options
Some(42)
Construct a non empty optional value.
None
The singleton empty optional value.
Option(null) == None
Option(obj.unsafeMethod)
but
Some(null) != None
Null-safe optional value factory.
val optStr: Option[String] = None
same as
val optStr = Option.empty[String]
Explicit type for empty optional value.
Factory for empty optional value.
val name: Option[String] =
  request.getParameter("name")
val upper = name.map {
  _.trim
} filter {
  _.length != 0
} map {
  _.toUpperCase
}
println(upper.getOrElse(""))
Pipeline style.
val upper = for {
  name <- request.getParameter("name")
  trimmed <- Some(name.trim)
    if trimmed.length != 0
  upper <- Some(trimmed.toUpperCase)
} yield upper
println(upper.getOrElse(""))
For-comprehension syntax.
option.map(f(_))
same as
option match {
  case Some(x) => Some(f(x))
  case None    => None
}
Apply a function on the optional value.
option.flatMap(f(_))
same as
option match {
  case Some(x) => f(x)
  case None    => None
}
Same as map but function must return an optional value.
optionOfOption.flatten
same as
optionOfOption match {
  case Some(Some(x)) => Some(x)
  case _             => None
}
Extract nested option.
option.foreach(f(_))
same as
option match {
  case Some(x) => f(x)
  case None    => ()
}
Apply a procedure on optional value.
option.fold(y)(f(_))
same as
option match {
  case Some(x) => f(x)
  case None    => y
}
Apply function on optional value, return default if empty.
option.collect {
  case x => ...
}
same as
option match {
  case Some(x) if f.isDefinedAt(x) => ...
  case Some(_)                     => None
  case None                        => None
}
Apply partial pattern match on optional value.
option.isDefined
same as
option match {
  case Some(_) => true
  case None    => false
}
true if not empty.
option.isEmpty
same as
option match {
  case Some(_) => false
  case None    => true
}
true if empty.
option.nonEmpty
same as
option match {
  case Some(_) => true
  case None    => false
}
true if not empty.
option.size
same as
option match {
  case Some(_) => 1
  case None    => 0
}
0 if empty, otherwise 1.
option.orElse(Some(y))
same as
option match {
  case Some(x) => Some(x)
  case None    => Some(y)
}
Evaluate and return alternate optional value if empty.
option.getOrElse(y)
same as
option match {
  case Some(x) => x
  case None    => y
}
Evaluate and return default value if empty.
option.get
same as
option match {
  case Some(x) => x
  case None    => throw new Exception
}
Return value, throw exception if empty.
option.orNull
same as
option match {
  case Some(x) => x
  case None    => null
}
Return value, null if empty.
option.filter(f)
same as
option match {
  case Some(x) if f(x) => Some(x)
  case _               => None
}
Optional value satisfies predicate.
option.filterNot(f(_))
same as
option match {
  case Some(x) if !f(x) => Some(x)
  case _                => None
}
Optional value doesn't satisfy predicate.
option.exists(f(_))
same as
option match {
  case Some(x) if f(x) => true
  case Some(_)         => false
  case None            => false
}
Apply predicate on optional value or false if empty.
option.forall(f(_))
same as
option match {
  case Some(x) if f(x) => true
  case Some(_)         => false
  case None            => true
}
Apply predicate on optional value or true if empty.
option.contains(y)
same as
option match {
  case Some(x) => x == y
  case None    => false
}
Checks if value equals optional value or false if empty.

Contributors to this page: