Scala: The Pointy Bits

The pared down version of "Scala: The Swiss Army Language" encompassing just the parts that could get you into trouble on day 1 of a dive into an existing, mature Scala codebase.

Transcript

1. the pointy bits SCALA: NATHAN DOTZ • @NATHANDOTZ • SELF.CONF 2015 SLEEPYNATE
2. 1/2 A BOOK
3. 1/2 A BOOK
4. 4-HOUR WORKSHOP
5. 4-HOUR WORKSHOP
6. ~45 MINUTE TALK
7. HOLD ON TO YOUR BUTTS
8. WHY SCALA? because it’s amazing.
10. override def ++[B1 >: B](xs: GenTraversableOnce[(A, B1)]): ListMap[A, B1] = ((repr: ListMap[A, B1]) /: xs.seq) (_ + _)
12. what does scala look like? A FIRST LOOK
13. DECLARE THINGS • val • var • def
14. scala> val s = "Hello Scala" s: String = Hello Scala VAL
15. scala> s = "reassignment" <console>:8: error: reassignment to val s = "reassignment" ^ VAL
16. scala> var t = "Hello variables" t: String = Hello variables scala> t = "Hello reassignment" t: String = Hello reassignment VAR
17. scala> def f = "Hello definitions" f: String
18. scala> def f = "Hello definitions" f: String DEF
19. scala> :type f String
21. scala> var x = 5 x: Int = 5 scala> def g = x g: Int scala> g res1: Int = 5 scala> x = 6 x: Int = 6 scala> g res2: Int = 6
22. scala> def exclaim(s: String) = s + "!!!" exclaim: (s: String)String scala> exclaim("Hello, Scala") res3: String = Hello, Scala!!!
23. scala> def exclaim(s: String):String = s + "!!!" SIGNATURES
24. scala> def exclaim(s: String):String = s + "!!!" SIGNATURES takes a String
25. scala> def exclaim(s: String):String = s + "!!!" SIGNATURES takes a String returns a String
26. scala> def exclaim(s: String):String = s + "!!!" EXPRESSION-BASED returns a String }
27. def exclaim(s:String) = { println(s) s + "!!!" } CURLY = BLOCK
28. def exclaim(s:String) = { println(s) s + "!!!" } CURLY = BLOCK
29. def exclaim(s:String) = { println(s) s + "!!!" } exclaim: (s: String)String CURLY = BLOCK
30. def exclaim = { s:String => println(s) s + "!!!" } exclaim: String => String CURLY = BLOCK
31. scala> def exclaim(s:String) = { | println(s) | s + "!!!" | } exclaim: (s: String)String scala> { s: String => println(s); s + "!!!" } res0: String => String = <function1> scala> exclaim("Hello, Scala") Hello, Scala res1: String = Hello, Scala!!! scala> res0("Hello, Scala") Hello, Scala res2: String = Hello, Scala!!!
32. scala> { s: String => println(s); s + "!!!" } res0: String => String = <function1> scala> exclaim("Hello, Scala") Hello, Scala res1: String = Hello, Scala!!! scala> res0("Hello, Scala") Hello, Scala res2: String = Hello, Scala!!! FAT ARROW = FUNCTION
33. scala> { s: String => println(s); s + "!!!" } res0: String => String = <function1> scala> exclaim("Hello, Scala") Hello, Scala res1: String = Hello, Scala!!! scala> res0("Hello, Scala") Hello, Scala res2: String = Hello, Scala!!! FAT ARROW = FUNCTION
34. scala> res2.toList res3: List[Char] = List(H, e, l, l, o, ,, , S, c, a, l, a, !, !, !) TYPE PARAMETERS a List of Chars
35. scala> val s = "Hello, Scala" s: String = Hello, Scala scala> s + "!!!" == s.+("!!!") res0: Boolean = true DSL SYNTAX
36. scala> val s = "Hello, Scala" s: String = Hello, Scala scala> s + "!!!" == s.+("!!!") res0: Boolean = true DSL SYNTAX
37. FAMILIAR GROUND how is this like what I know already?
38. val s = "Hello Scala" var t = "Hello variables" final String s = "Hello Scala"; String t = "Hello variables";
39. def f = "Hello definitions" String f() { return "Hello definitions"; }
40. var x = 5 def g = x g x = 6 g int x = 5; int getX() { return x; } getX(); x = 6; getX();
41. def exclaim(s: String) = s + "!!!" String exclaim(String s) { return s + "!!!"; }
42. def exclaim(s:String) = { println(s) s + "!!!" } String exclaim(String s) { System.out.println(s); return s + "!!!"; }
43. scala> val f = { s: String => s + "!!!" } f: String => String = <function1> scala> f("hi") res0: String = hi!!!
44. scala> val f = { s: String => s + "!!!" } f: String => String = <function1> scala> f("hi") res0: String = hi!!! scala> { s: String => s + "!!!" }.apply("Hello, Scala") res1: String = Hello, Scala!!!
45. scala> { s: String => s + "!!!" }.apply("Hello, Scala") res1: String = Hello, Scala!!! public abstract class Function1<T, R> { public R apply(T t); } final Function1<String, String> f = new Function1<String, String>() { public String apply(String s) { return s + "!!!"; } }; f.apply(”Hello, Scala");
46. scala> { s: String => s + "!!!" }.apply("Hello, Scala") res1: String = Hello, Scala!!! (s -> s + "!!!")("Hello, Scala");
47. List<Integer> l = new List<Integer>(); l.addAll(Arrays.asList(1,2,3,4)); scala> val l = List(1,2,3,4) l: List[Int] = List(1, 2, 3, 4)
48. "Hello, Java".split(", "); scala> "Hello, Scala" split ", " res2: Array[String] = Array(Hello, Scala)
49. scala> "Hello, Scala" split ", " length <console>:8: warning: postfix operator length should be enabled by making the implicit value scala.language.postfixOps visible. This can be achieved by adding the import clause 'import scala.language.postfixOps'
50. scala> ("Hello, Scala" split ", ") length <console>:8: warning: postfix operator length should be enabled by making the implicit value scala.language.postfixOps visible. This can be achieved by adding the import clause 'import scala.language.postfixOps'
51. scala> import scala.language.postfixOps import scala.language.postfixOps scala> "Hello, Scala" split ", " length res2: Int = 2
52. what does scala do better? IMPROVEMENTS
53. scala> Array(2, 3, 5, 8, 13, 21, 43) res0: Array[Int] = Array(2, 3, 5, 8, 13, 21, 43) scala> res0.map(x => x * 2) res1: Array[Int] = Array(4, 6, 10, 16, 26, 42, 86) scala> res0.filter { x => x % 2 == 0 } res2: Array[Int] = Array(2, 8) scala> res0.toList res3: List[Int] = List(2, 3, 5, 8, 13, 21, 43) COLLECTIONS
54. scala> res3 res4: List[Int] = List(2, 3, 5, 8, 13, 21, 43) scala> res3.map(x => x * 2) res5: List[Int] = List(4, 6, 10, 16, 26, 42, 86) scala> res3.filter { x => x % 2 == 0 } res6: List[Int] = List(2, 8) scala> res3.toSet res7: Set[Int] = Set(5, 21, 13, 2, 3, 43, 8) BATTERIES INCLUDED
55. scala> res7 res8: Set[Int] = Set(5, 21, 13, 2, 3, 43, 8) scala> res7.map(x => x * 2) res9: Set[Int] = Set(10, 42, 6, 86, 16, 26, 4) scala> res7.filter { x => x % 2 == 0 } res10: Set[Int] = Set(2, 8) scala> res7.toStream res11: Stream[Int] = Stream(5, ?) HECK YEA!
57. final class Vector[+A] private[immutable] (private[collection] val startIndex: Int, private[collection] val endIndex: Int, focus: Int) extends AbstractSeq[A] with IndexedSeq[A] with GenericTraversableTemplate[A, Vector] with IndexedSeqLike[A, Vector[A]] with VectorPointer[A @uncheckedVariance] with Serializable with CustomParallelizable[A, ParVector[A]] sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A, List] with LinearSeqOptimized[A, List[A]] with Serializable { class ListMap[A, +B] extends AbstractMap[A, B] with Map[A, B] with MapLike[A, B, ListMap[A, B]] with Serializable {
58. final def map[B](f: (A) => B): List[B] final def map[B, That](f: (A) => B)(implicit bf: CanBuildFrom[List[A], B, That]): That
59. case class Person(name: String, age: Int) CASE CLASSES
60. public class Person { private final String name; private final int age; public Person(final String name, final Integer age) { if(age == null || name == null) throw new NullPointerException(); this.name = name; this.age = age; } public String name() { return name; } public int age() { return age; } public Person copy(String name, Integer age) { return apply(name == null ? this.name : name, age == null ? this.age : age); } public Person copy(String name) { return apply(name == null ? this.name : name, this.age); } public Person copy(Integer age) { return apply(this.name, age == null ? this.age : age); } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + age; result = prime * result + ((name == null) ? 0 : name.hashCode()); return result; } @Override public boolean equals(Object obj) { if (this == obj) { return true; } if (obj == null) { return false; } if (getClass() != obj.getClass()) { return false; } Person other = (Person) obj; if (age != other.age) { return false; } if (name == null && other.name != null) { return false; } else if (!name.equals(other.name)) { return false; } return true; } @Override public String toString() { return "Person(name=" + name + ", age=" + age + ")"; } public static Person apply(String name, int age) { return new Person(name, age); } }
61. public class Person { private final String name; private final int age; public Person(final String name, final Integer age) { if(age == null || name == null) throw new NullPointerException(); this.name = name; this.age = age; } public String name() { return name; } public int age() { return age; }
62. public Person copy(String name, Integer age) { return apply(name == null ? this.name : name, age == null ? this.age : age); } public Person copy(String name) { return apply(name == null ? this.name : name, this.age); } public Person copy(Integer age) { return apply(this.name, age == null ? this.age : age); }
63. public static Person apply(String name, int age) { return new Person(name, age); }
64. @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Person other = (Person) obj; if (age != other.age) return false; if (name == null && other.name != null) return false; else if (!name.equals(other.name)) return false; return true; }
65. @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + age; result = prime * result + ((name == null) ? 0 : name.hashCode()); return result; } @Override public String toString() { return "Person(name=" + name + ", age=" + age + ")"; }
66. package com.velvet.model; public class Person { private final String name; private final int age; public Person(final String name, final Integer age) { if(age == null || name == null) throw new NullPointerException(); this.name = name; this.age = age; } public String name() { return name; } public int age() { return age; } public Person copy(String name, Integer age) { return new Person(name == null ? this.name : name, age == null ? this.age : age); } public Person copy(String name) { return new Person(name == null ? this.name : name, this.age); } public Person copy(Integer age) { return new Person(this.name, age == null ? this.age : age); } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + age; result = prime * result + ((name == null) ? 0 : name.hashCode()); return result; } @Override public boolean equals(Object obj) { if (this == obj) { return true; } if (obj == null) { return false; } if (getClass() != obj.getClass()) { return false; } Person other = (Person) obj; if (age != other.age) { return false; } if (name == null && other.name != null) { return false; } else if (!name.equals(other.name)) { return false; } return true; } @Override public String toString() { return "Person(name=" + name + ", age=" + age + ")"; } public static Person apply(String name, int age) { return new Person(name, age); } } case class Person(name: String, age: Int)
67. trait Logger { val leftSep = "["; val rightSep = "]" protected def print(message: String) def error(message: String) = { print(s"${leftSep}error${rightSep} $message") } } TRAITS
68. class ConsoleLogger extends Logger { protected def print(message: String) { println(message) } } TRAITS
70. sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A, List] with LinearSeqOptimized[A, List[A]] with Serializable { MULTIPLE INHERITANCE
71. scala> new Person("dotz", 32) with ConsoleLogger res14: Person with ConsoleLogger = Person(dotz,32) scala> res14 error "derp" [error] derp TRAITS
72. scala> import java.util.Calendar import java.util.Calendar scala> lazy val lazyTime = Calendar.getInstance.getTimeInMillis lazyTime: Long = <lazy> scala> Calendar.getInstance.getTimeInMillis res0: Long = 1431905681166 scala> lazyTime res1: Long = 1431905685466 LAZY VALS
73. class UnixTime(val seconds: Long) object UnixTime { def now = new UnixTime(Calendar.getInstance.getTimeInMillis / 1000) } OBJECTS
74. scala> classOf[UnixTime] res2: Class[UnixTime] = class UnixTime scala> UnixTime.getClass res3: Class[_ <: UnixTime.type] = class UnixTime$ OBJECTS
75. def fibonacci(n: Int):Int = {
 def fib(n: Int, curr: Int, acc: Int):Int =
 if (n == 0) acc
 else fib(n - 1, acc, curr + acc) 
 fib(n, 1, 0)
 } TAIL-CALL RECURSION
76. def factorial(n:Int):Int = n match { case 0 => 1 case _ => n * factorial(n-1) } TAIL-CALL RECURSION
77. @tailrec def factorial(n:Int):Int = n match { case 0 => 1 case _ => n * factorial(n-1) } TAIL-CALL RECURSION <console>:11: error: could not optimize @tailrec annotated method factorial: it contains a recursive call not in tail position case _ => n * factorial(n-1)
78. NEW TOYS what haven’t I seen before?
79. PATTERN MATCHING def magicNumber(n: Int) = n match { case 3 => "IT'S THE MAGIC NUMBER!" case _ => "just an ordinary number" }
80. scala> List(1,2,3,4) match { | case head :: tail => (head, tail) | } res0: (Int, List[Int]) = (1,List(2, 3, 4)) PATTERN MATCHING
81. scala> List(1,2,3,4) match { | case head :: tail => (head, tail) | } <console>:8: warning: match may not be exhaustive. It would fail on the following input: Nil List(1,2,3,4) match { ^ res0: (Int, List[Int]) = (1,List(2, 3, 4)) PATTERN MATCHING
82. List(1,2,3,4) match { case head :: tail => (head, tail) case _ => (0, None) } PATTERN MATCHING
83. scala> List(1,2,3,4) match { | case head :: tail => (head, tail) | case _ => (0, None) | } res1: (Int, Product with java.io.Serializable) = (1,List(2, 3, 4)) PATTERN MATCHING
84. "bar" match { case "bar" => "matches!" } PATTERN MATCHING
85. val BigFoo = "foo" "bar" match { case BigFoo => "doesn't match" } PATTERN MATCHING
86. val foo = "foo" "bar" match { case foo => "this also matches! } PATTERN MATCHING val foo = "foo" "bar" match { case foo => "this also matches! } different "foo"s
87. scala> val x = 5 x: Int = 5 scala> def f(implicit i: Int) = i + 5 f: (implicit i: Int)Int scala> f <console>:9: error: could not find implicit value for parameter i: Int f ^ scala> implicit val x = 5 x: Int = 5 scala> f res3: Int = 10 IMPLICIT VALUES
88. scala> import scala.language.{postfixOps, implicitConversions, reflectiveCalls} import scala.language.{postfixOps, implicitConversions, reflectiveCalls} scala> implicit def makeFooable(s: String) = new Object { def isFoo = s == "foo" } makeFooable: (s: String)Object{def isFoo: Boolean} scala> "abc" isFoo res0: Boolean = false scala> "foo" isFoo res1: Boolean = true IMPLICIT FUNCTIONS
89. scala> import scala.language.implicitConversions import scala.language.implicitConversions scala> implicit def doubleToInt(x: Double) = x.toInt doubleToInt: (x: Double)Int scala> val i: Int = 3.5 i: Int = 3 IMPLICIT FUNCTIONS
90. scala> implicit class FooableString(s: String) { | def isFoo = s == "foo" | } defined class FooableString scala> "foo" isFoo res1: Boolean = true IMPLICIT CLASSES
91. trait Mappable[T[_]] { def map[A,B](x: T[A])(f: A => B):T[B] } class Stupid[+A](val a: A) object Stupid { def apply[A](x: => A) = new Stupid[A](x) } implicit object StupidMappable extends Mappable[Stupid] { override def map[A, B](x: Stupid[A])(f: (A) => B) = new Stupid[B](f(x.a)) } implicit def makeMappableATypeClass[S[_] : Mappable, A](stupidThing: S[A]) = new { val functor = implicitly[Mappable[S]] final def map[B](f: A => B): S[B] = functor.map(stupidThing)(f) } TYPE CLASSES
92. Given f(x, y) = z Then curry(f) = (x => (y => z)) CURRYING
93. scala> def add(x: Int)(y: Int) = x + y add: (x: Int)(y: Int)Int scala> add(2) _ res0: Int => Int = <function1> scala> res0(3) res1: Int = 5 CURRYING
94. scala> def add2(x: Int, y: Int) = x + y add2: (x: Int, y: Int)Int scala> (add2 _).curried res0: Int => (Int => Int) = <function1> CURRYING