Scala features cheatsheet

gistfile1.md

Cheat Sheet

This cheat sheet originated from the forum, credits to Laurent Poulain. We copied it and changed or added a few things.

Evaluation Rules

• Call by value: evaluates the function arguments before calling the function
• Call by name: evaluates the function first, and then evaluates the arguments if need be

def example = 2      // evaluated when called
val example = 2      // evaluated immediately
lazy val example = 2 // evaluated once when needed

def square(x: Double)    // call by value
def square(x: => Double) // call by name
def myFct(bindings: Int*) { ... } // bindings is a sequence of int, containing a varying # of arguments

Higher order functions

These are functions that take a function as a parameter or return functions.

// sum() returns a function that takes two integers and returns an integer  
def sum(f: Int => Int): (Int, Int) => Int = {  
  def sumf(a: Int, b: Int): Int = {...}  
  sumf  
} 

// same as above. Its type is (Int => Int) => (Int, Int) => Int  
def sum(f: Int => Int)(a: Int, b: Int): Int = { ... } 

// Called like this
sum((x: Int) => x * x * x)          // Anonymous function, i.e. does not have a name  
sum(x => x * x * x)                 // Same anonymous function with type inferred

def cube(x: Int) => x * x * x  
sum(x => x * x * x)(1, 10) // sum of cubes from 1 to 10
sum(cube)(1, 10)           // same as above      

Currying

Converting a function with multiple arguments into a function with a single argument that returns another function.

def f(a: Int, b: Int): Int // uncurried version (type is (Int, Int) => Int)
def f(a: Int)(b: Int): Int // curried version (type is Int => Int => Int)

Classes

class MyClass(x: Int, y: Int) {           // Defines a new type MyClass with a constructor  
  require(y > 0, "y must be positive")    // precondition, triggering an IllegalArgumentException if not met  
  def this (x: Int) = { ... }             // auxiliary constructor   
  def nb1 = x                             // public method computed every time it is called  
  def nb2 = y  
  private def test(a: Int): Int = { ... } // private method  
  val nb3 = x + y                         // computed only once  
  override def toString =                 // overridden method  
      member1 + ", " + member2 
  }

new MyClass(1, 2) // creates a new object of type

this references the current object, assert(<condition>) issues AssertionError if condition is not met. See scala.Predef for require, assume and assert.

Operators

myObject myMethod 1 is the same as calling myObject.myMethod(1)

Operator (i.e. function) names can be alphanumeric, symbolic (e.g. x1, *, +?%&, vector_++, counter_=)

The precedence of an operator is determined by its first character, with the following increasing order of priority:

(all letters)
|
^
&
< >
= !
:
+ -
* / %
(all other special characters)

The associativity of an operator is determined by its last character: Right-associative if ending with :, Left-associative otherwise.

Note that assignment operators have lowest precedence. (Read Scala Language Specification 2.9 sections 6.12.3, 6.12.4 for more info)

Class hierarchies

abstract class TopLevel {     // abstract class  
  def method1(x: Int): Int   // abstract method  
  def method2(x: Int): Int = { ... }  
}

class Level1 extends TopLevel {  
  def method1(x: Int): Int = { ... }  
  override def method2(x: Int): Int = { ...} // TopLevel's method2 needs to be explicitly overridden  
}

object MyObject extends TopLevel { ... } // defines a singleton object. No other instance can be created

To create an runnable application in Scala:

object Hello {  
  def main(args: Array[String]) = println("Hello world")  
}

or

object Hello extends App {
  println("Hello World")
}

Class Organization

• Classes and objects are organized in packages (package myPackage).

• They can be referenced through import statements (import myPackage.MyClass, import myPackage._, import myPackage.{MyClass1, MyClass2}, import myPackage.{MyClass1 => A})

• They can also be directly referenced in the code with the fully qualified name (new myPackage.MyClass1)

• All members of packages scala and java.lang as well as all members of the object scala.Predef are automatically imported.

• Traits are similar to Java interfaces, except they can have non-abstract members:

    trait Planar { ... }
    class Square extends Shape with Planar
  

• General object hierarchy:

  • scala.Any base type of all types. Has methods hashCode and toString that can be overloaded
  • scala.AnyVal base type of all primitive types. (scala.Double, scala.Float, etc.)
  • scala.AnyRef base type of all reference types. (alias of java.lang.Object, supertype of java.lang.String, scala.List, any user-defined class)
  • scala.Null is a subtype of any scala.AnyRef (null is the only instance of type Null), and scala.Nothing is a subtype of any other type without any instance.

Type Parameters

Similar to C++ templates or Java generics. These can apply to classes, traits or functions.

class MyClass[T](arg1: T) { ... }  
new MyClass[Int](1)  
new MyClass(1)   // the type is being infered, i.e. determined based on the value arguments  

It is possible to restrict the type being used, e.g.

def myFct[T <: TopLevel](arg: T): T = { ... } // T must derive from TopLevel or be TopLevel
def myFct[T >: Level1](arg: T): T = { ... }   // T must be a supertype of Level1
def myFct[T >: Level1 <: Top Level](arg: T): T = { ... }

Variance

Given A <: B

If C[A] <: C[B], C is covariant

If C[A] >: C[B], C is contravariant

Otherwise C is nonvariant

class C[+A] { ... } // C is covariant
class C[-A] { ... } // C is contravariant
class C[A]  { ... } // C is nonvariant

For a function, if A2 <: A1 and B1 <: B2, then A1 => B1 <: A2 => B2.

Functions must be contravariant in their argument types and covariant in their result types, e.g.

trait Function1[-T, +U] {
  def apply(x: T): U
} // Variance check is OK because T is contravariant and U is covariant

class Array[+T] {
  def update(x: T)
} // variance checks fails

Find out more about variance in lecture 4.4 and lecture 4.5

Pattern Matching

Pattern matching is used for decomposing data structures:

unknownObject match {
  case MyClass(n) => ...
  case MyClass2(a, b) => ...
}

Here are a few example patterns

(someList: List[T]) match {
  case Nil => ...          // empty list
  case x :: Nil => ...     // list with only one element
  case List(x) => ...      // same as above
  case x :: xs => ...      // a list with at least one element. x is bound to the head,
                           // xs to the tail. xs could be Nil or some other list.
  case 1 :: 2 :: cs => ... // lists that starts with 1 and then 2
  case (x, y) :: ps => ... // a list where the head element is a pair
}

The last example shows that every pattern consists of sub-patterns: it only matches lists with at least one element, where that element is a pair. x and y are again patterns that could match only specific types.

Pattern matches are also used quite often in anonymous functions:

val pairs: List[(Char, Int)] = ('a', 2) :: ('b', 3) :: Nil
val chars: List[Char] = pairs.map(p => p match {
  case (ch, num) => ch
})

Instead of p => p match { case ... }, you can simply write {case ...}, so the above example becomes more concise:

val chars: List[Char] = pairs map {
  case (ch, num) => ch
}

Collections

Scala defines several collection classes:

Base Classes

• Iterable (collections you can iterate on)
• Seq (ordered sequences)
• Set
• Map (lookup data structure)

Immutable Collections

• List (linked list, provides fast sequential access)
• Vector (array-like type, implemented as tree of blocks, provides fast random access)
• Range (ordered sequence of integers with equal spacing)
• String (Java type, implicitly converted to a character sequence, so you can treat every string like a Seq[Char])
• Map (collection that maps keys to values)
• Set (collection without duplicate elements)

Mutable Collections

• Array (Scala arrays are native JVM arrays at runtime, therefore they are very performant)
• Scala also has mutable maps and sets; these should only be used if there are performance issues with immutable types

Examples

val fruitList = List("apples", "oranges", "pears")
// Alternative syntax for lists
val fruit = "apples" :: ("oranges" :: ("pears" :: Nil)) // parens optional, :: is right-associative
fruit.head   // "apples"
fruit.tail   // List("oranges", "pears")
val empty = List()
val empty = Nil

val nums = Vector("louis", "frank", "hiromi")
nums(1)                     // element at index 1, returns "frank", complexity O(log(n))
nums.updated(2, "helena")   // new vector with a different string at index 2, complexity O(log(n))

val fruitSet = Set("apple", "banana", "pear", "banana")
fruitSet.size    // returns 3: there are no duplicates, only one banana

val r: Range = 1 until 5 // 1, 2, 3, 4
val s: Range = 1 to 5    // 1, 2, 3, 4, 5
1 to 10 by 3  // 1, 4, 7, 10
6 to 1 by -2  // 6, 4, 2

val s = (1 to 6).toSet
s map (_ + 2) // adds 2 to each element of the set

val s = "Hello World"
s filter (c => c.isUpper) // returns "HW"; strings can be treated as Seq[Char]

// Operations on sequences
val xs = List(...)
xs.length   // number of elements, complexity O(n)
xs.last     // last element (exception if xs is empty), complexity O(n)
xs.init     // all elements of xs but the last (exception if xs is empty), complexity O(n)
xs take n   // first n elements of xs
xs drop n   // the rest of the collection after taking n elements
xs(n)       // the nth element of xs, complexity O(n)
xs ++ ys    // concatenation, complexity O(n)
xs.reverse  // reverse the order, complexity O(n)
xs updated(n, x)  // same list than xs, except at index n where it contains x, complexity O(n)
xs indexOf x      // the index of the first element equal to x (-1 otherwise)
xs contains x     // same as xs indexOf x >= 0
xs filter p       // returns a list of the elements that satisfy the predicate p
xs filterNot p    // filter with negated p 
xs partition p    // same as (xs filter p, xs filterNot p)
xs takeWhile p    // the longest prefix consisting of elements that satisfy p
xs dropWhile p    // the remainder of the list after any leading element satisfying p have been removed
xs span p         // same as (xs takeWhile p, xs dropWhile p)

List(x1, ..., xn) reduceLeft op    // (...(x1 op x2) op x3) op ...) op xn
List(x1, ..., xn).foldLeft(z)(op)  // (...( z op x1) op x2) op ...) op xn
List(x1, ..., xn) reduceRight op   // x1 op (... (x{n-1} op xn) ...)
List(x1, ..., xn).foldRight(z)(op) // x1 op (... (    xn op  z) ...)

xs exists p    // true if there is at least one element for which predicate p is true
xs forall p    // true if p(x) is true for all elements
xs zip ys      // returns a list of pairs which groups elements with same index together
xs unzip       // opposite of zip: returns a pair of two lists
xs.flatMap f   // applies the function to all elements and concatenates the result
xs.sum         // sum of elements of the numeric collection
xs.product     // product of elements of the numeric collection
xs.max         // maximum of collection
xs.min         // minimum of collection
xs.flatten     // flattens a collection of collection into a single-level collection
xs groupBy f   // returns a map which points to a list of elements
xs distinct    // sequence of distinct entries (removes duplicates)

x +: xs  // creates a new collection with leading element x
xs :+ x  // creates a new collection with trailing element x

// Operations on maps
val myMap = Map("I" -> 1, "V" -> 5, "X" -> 10)  // create a map
myMap("I")      // => 1  
myMap("A")      // => java.util.NoSuchElementException  
myMap get "A"   // => None 
myMap get "I"   // => Some(1)
myMap.updated("V", 15)  // returns a new map where "V" maps to 15 (entry is updated)
                        // if the key ("V" here) does not exist, a new entry is added

Pairs (similar for larger Tuples)

val pair = ("answer", 42)   // type: (String, Int)
val (label, value) = pair   // label = "answer", value = 42  
pair._1 // "answer"  
pair._2 // 42  

Ordering

There is already a class in the standard library that represents orderings: scala.math.Ordering[T] which contains comparison functions such as lt() and gt() for standard types. Types with a single natural ordering should inherit from the trait scala.math.Ordered[T].

import math.Ordering  

def msort[T](xs: List[T])(implicit ord: Ordering) = { ...}  
msort(fruits)(Ordering.String)  
msort(fruits)   // the compiler figures out the right ordering  

For-Comprehensions

A for-comprehension is syntactic sugar for map, flatMap and filter operations on collections.

The general form is for (s) yield e

• s is a sequence of generators and filters
• p <- e is a generator
• if f is a filter
• If there are several generators (equivalent of a nested loop), the last generator varies faster than the first
• You can use { s } instead of ( s ) if you want to use multiple lines without requiring semicolons
• e is an element of the resulting collection

Example 1

// list all combinations of numbers x and y where x is drawn from
// 1 to M and y is drawn from 1 to N
for (x <- 1 to M; y <- 1 to N)
  yield (x,y)

is equivalent to

(1 to M) flatMap (x => (1 to N) map (y => (x, y)))

Translation Rules

A for-expression looks like a traditional for loop but works differently internally

for (x <- e1) yield e2 is translated to e1.map(x => e2)

for (x <- e1 if f) yield e2 is translated to for (x <- e1.filter(x => f)) yield e2

for (x <- e1; y <- e2) yield e3 is translated to e1.flatMap(x => for (y <- e2) yield e3)

This means you can use a for-comprehension for your own type, as long as you define map, flatMap and filter.

For more, see lecture 6.5.

Example 2

for {  
  i <- 1 until n  
  j <- 1 until i  
  if isPrime(i + j)  
} yield (i, j)  

is equivalent to

for (i <- 1 until n; j <- 1 until i if isPrime(i + j))
    yield (i, j)  

is equivalent to

(1 until n).flatMap(i => (1 until i).filter(j => isPrime(i + j)).map(j => (i, j)))