Scalacheat

Contributed by Brendan O'Connor

variables
var x = 5	variable
Good 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 Unit-returning procedure; causes havoc
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.
Good (1 to 5).map(2*) Bad (1 to 5).map(*2)	anonymous function: bound infix method. Use 2*_ for sanity's sake instead.
(1 to 5).map { val x=_*2; println(x); x }	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.
package pkg at start of file package 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.
Badvar 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)	sole member of the Unit type (like C/Java void).
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) { println("%d/%d = %.1f".format(x,y, x*y)) }	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 Some(3) match { case Some(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 Some(3) match { case Some(`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 Some(3) match { case Some(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) same as class C(private val x: R) var c = new C(4)	constructor params - private
class C(val x: R) var c = new C(4) c.x	constructor params - public
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)