Scala
Puzz ers
Andrew Phillips & Nermin Šerifovic
Scala
Puzz ers
Andrew Phillips & Nermin Šerifovic
Scala
Puzz ers
Andrew Phillips & Nermin Šerifovic
What is a Puzzler?
● 
Intentional language feature
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What is a Puzzler?
● 
Intentional language feature
● 
So no bugs
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What is a Puzzler?
● 
Intentional language feature
● 
● 
So no bugs
Not just weird because you don't know Scala
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What is a Puzzler?
● 
Intentional language feature
● 
● 
So no bugs
Not just weird because you don't know Scala
● 
So no "what is this weird operator?!?"
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What is a Puzzler?
● 
Intentional language feature
● 
● 
Not just weird because you don't know Scala
● 
● 
So no bugs
So no "what is this weird operator?!?"
Should be "surprising" or "non-intuitive" to a
reasonably skilled Scala developer
www.scalapuzzlers.com
What is a Puzzler?
● 
Intentional language feature
● 
● 
Not just weird because you don't know Scala
● 
● 
So no bugs
So no "what is this weird operator?!?"
Should be "surprising" or "non-intuitive" to a
reasonably skilled Scala developer
● 
For suitable definitions of "reasonably skilled"
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What is a Puzzler?
● 
Tested against the Scala 2.11.2 REPL
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It'll be More Fun if...
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Having Said That...
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Init You, Init Me
object XY {
object X { val value: Int = Y.value + 1 }
object Y { val value: Int = X.value + 1 }
}
println(if (math.random > 0.5) XY.X.value else XY.Y.value)
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Init You, Init Me
object XY {
object X { val value: Int = Y.value + 1 }
object Y { val value: Int = X.value + 1 }
}
println(if (math.random > 0.5) XY.X.value else XY.Y.value)
A. Prints
1
B. Prints
2
C. Prints either
1
or
2
D. Throws a runtime exception
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Init You, Init Me
object XY {
object X { val value: Int = Y.value + 1 }
object Y { val value: Int = X.value + 1 }
}
println(if (math.random > 0.5) XY.X.value else XY.Y.value)
A. Prints
1
B. Prints
2
C. Prints either
1
or
2
D. Throws a runtime exception
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Well, In This Case...
case class Toy(squeezeMsg: String = this.toString) {
override def toString = squeezeMsg
}
println(Toy("My name is Fido!") == new Toy("My name is Fido!"))
println(Toy() == new Toy())
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Well, In This Case...
case class Toy(squeezeMsg: String = this.toString) {
override def toString = squeezeMsg
}
println(Toy("My name is Fido!") == new Toy("My name is Fido!"))
println(Toy() == new Toy())
A. Fails to compile
B. Prints
true
false
C. Prints
D. The first statement prints
true
and the second throws a runtime
exception
true
true
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Well, In This Case...
case class Toy(squeezeMsg: String = this.toString) {
override def toString = squeezeMsg
}
println(Toy("My name is Fido!") == new Toy("My name is Fido!"))
println(Toy() == new Toy())
A. Fails to compile
B. Prints
true
false
C. Prints
D. The first statement prints
true
and the second throws a runtime
exception
true
true
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Trial and Error
def divideThenSqrt(n: Int, d: Int) =
Try(n / d) map {
value => if (value < 0)
Failure(new ArithmeticException(
"square root of negative number"))
else Success(math.sqrt(value))
} map {
value => s"sqrt(${n} / ${d}) = ${value}”
}
println(divideThenSqrt(18, 2))
println(divideThenSqrt(18, -2))
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Trial and Error
A. Prints
B. Prints
Success(sqrt(18 / 2) = 3.0)
Success(sqrt(18 / 2) = 3.0)
Success(java.lang.ArithmeticE
xception: square root of
negative number)
Failure(java.lang.ArithmeticE
xception: square root of
negative number)
C. Prints
D. Prints
Success(sqrt(18 / 2) =
Success(3.0))
Success(sqrt(18 / -2) =
Failure(java.lang.ArithmeticE
xception: square root of
negative number))
Success(sqrt(18 / 2) = 3.0)
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Success(sqrt(18 / -2) =
java.lang.ArithmeticException
: square root of negative
number)
Trial and Error
A. Prints
B. Prints
Success(sqrt(18 / 2) = 3.0)
Success(sqrt(18 / 2) = 3.0)
Success(java.lang.ArithmeticE
xception: square root of
negative number)
Failure(java.lang.ArithmeticE
xception: square root of
negative number)
C. Prints
D. Prints
Success(sqrt(18 / 2) =
Success(3.0))
Success(sqrt(18 / -2) =
Failure(java.lang.ArithmeticE
xception: square root of
negative number))
Success(sqrt(18 / 2) = 3.0)
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Success(sqrt(18 / -2) =
java.lang.ArithmeticException
: square root of negative
number)
Partially Even
val isEven = PartialFunction[Int, String] {
case n if n % 2 == 0 => "Even"
}
val isOdd = PartialFunction[Int, String] {
case n if n % 2 == 1 => "Odd"
}
println(Seq(0, 2) map (isEven orElse isOdd))
println(Seq(0, 1, 2) map (isEven orElse isOdd))
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Partially Even
A. Prints
List(Even, Even)
List(Even, Even, Even)
B. Prints
List(Even, Even)
List(Even, Odd, Even)
C. Prints
D. The first statement prints
List(Even, Even)
and the second throws a runtime
exception
List("0", "2")
List("0", "1", "2")
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Partially Even
A. Prints
List(Even, Even)
List(Even, Even, Even)
B. Prints
List(Even, Even)
List(Even, Odd, Even)
C. Prints
D. The first statement prints
List(Even, Even)
and the second throws a runtime
exception
List("0", "2")
List("0", "1", "2")
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Information Overload
object Oh {
def overloadA(u: Unit) = "I accept a Unit"
def overloadA(u: Unit, n: Nothing) =
"I accept a Unit and Nothing"
def overloadB(n: Unit) = "I accept a Unit"
def overloadB(n: Nothing) = "I accept Nothing"
}
println(Oh overloadA 99)
println(Oh overloadB 99)
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Information Overload
A. The first statement prints
I accept a Unit
and the second fails to compile
B. The first statement fails to
compile and the second prints
I accept Nothing
C. Both statements fail to
compile
D. Prints
I accept a Unit
I accept a Unit
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Information Overload
A. The first statement prints
I accept a Unit
and the second fails to compile
B. The first statement fails to
compile and the second prints
I accept Nothing
C. Both statements fail to
compile
D. Prints
I accept a Unit
I accept a Unit
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Extract or Fail?
val n = null
val nObj: Object = null
println(n match { case Symbol(x) => x; case _ => "Not a symbol" })
println(nObj match { case Symbol(x) => x; case _ => "Not a symbol" })
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Extract or Fail?
val n = null
val nObj: Object = null
println(n match { case Symbol(x) => x; case _ => "Not a symbol" })
println(nObj match { case Symbol(x) => x; case _ => "Not a symbol" })
A. Prints
Not a symbol
Not a symbol
B. Both statements throw a
NullPointerException
C. Prints
null
Not a symbol
D. The first statement throws a
NullPointerException and
the second prints
Not a symbol
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Extract or Fail?
val n = null
val nObj: Object = null
println(n match { case Symbol(x) => x; case _ => "Not a symbol" })
println(nObj match { case Symbol(x) => x; case _ => "Not a symbol" })
A. Prints
Not a symbol
Not a symbol
B. Both statements throw a
NullPointerException
C. Prints
null
Not a symbol
D. The first statement throws a
NullPointerException and
the second prints
Not a symbol
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Think of 1 Card
case class Card(number: Int, suit: String = "clubs") {
val value = (number % 13) + 1 // ace = 1, king = 13
def isInDeck(implicit deck: List[Card]) = deck contains this
}
implicit val deck = List(Card(1, "spades"))
implicit def intToCard(n: Int) = Card(n)
println(1.isInDeck)
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Think of 1 Card
case class Card(number: Int, suit: String = "clubs") {
val value = (number % 13) + 1 // ace = 1, king = 13
def isInDeck(implicit deck: List[Card]) = deck contains this
}
implicit val deck = List(Card(1, "spades"))
implicit def intToCard(n: Int) = Card(n)
println(1.isInDeck)
A. Throws a runtime exception
B. Prints
false
C. Prints
D. Fails to compile
true
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Think of 1 Card
case class Card(number: Int, suit: String = "clubs") {
val value = (number % 13) + 1 // ace = 1, king = 13
def isInDeck(implicit deck: List[Card]) = deck contains this
}
implicit val deck = List(Card(1, "spades"))
implicit def intToCard(n: Int) = Card(n)
println(1.isInDeck)
A. Throws a runtime exception
B. Prints
false
C. Prints
D. Fails to compile
true
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Implicit Kryptonite
trait Messages { def greeting: String }
trait Printer { def print(msg: String) }
object NormalDay {
implicit val EverydayMessages =
new Messages { val greeting = "Hello, world!" }
implicit val ConsolePrinter = new Printer {
def print(msg: String) { println(msg) }
}
}
object SunnyDay {
implicit val HappyMessages =
new Messages { val greeting = "What a nice day!" }
implicit val ConsolePrinter = new Printer {
def print(msg: String) { println(s"$msg :-)") }
}
}
import NormalDay._
implicitly[Printer] print implicitly[Messages].greeting
{
import SunnyDay._
implicitly[Printer] print implicitly[Messages].greeting
}
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Implicit Kryptonite
A. The first statement prints
Hello, world!
and the second fails to compile
B. Prints
Hello, world!
What a nice day! :-)
C. Prints
Hello, world!
What a nice day!
D. Prints
Hello, world! :-)
What a nice day! :-)
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Implicit Kryptonite
A. The first statement prints
Hello, world!
and the second fails to compile
B. Prints
Hello, world!
What a nice day! :-)
C. Prints
Hello, world!
What a nice day!
D. Prints
Hello, world! :-)
What a nice day! :-)
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