First Things First

Thanks!

Slides prepared using replhtml from @retronym: https://gist.github.com/retronym/5520762

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What is a Puzzler?

• It has to cover an intentional language feature

What is a Puzzler?

• It has to cover an intentional language feature
  • So no bugs

What is a Puzzler?

• It has to cover an intentional language feature
  • So no bugs
• Not just weird because you don't know Scala

What is a Puzzler?

• It has to cover an intentional language feature
  • So no bugs
• Not just weird because you don't know Scala
  • So no "what is this weird operator?!?"

What is a Puzzler?

• It has to cover an intentional language feature
  • So no bugs
• Not just weird because you don't know Scala
  • So no "what is this weird operator?!?"
• Should be "surprising" or "non-intuitive" to
  a reasonably skilled Scala developer

What is a Puzzler?

• It has to cover an intentional language feature
  • So no bugs
• Not just weird because you don't know Scala
  • 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" :-)

It'll be More Fun if...

Buckle Up

Stringy Conversions

class A { implicit val stringToInt = (_: String).toInt println("4" - 2) } class B { implicit val stringToInt: String => Int = _.toInt println("4" - 2) } new A() new B()

Stringy Conversions

class A { implicit val stringToInt = (_: String).toInt println("4" - 2) } class B { implicit val stringToInt: String => Int = _.toInt println("4" - 2) } new A() new B()

1. Prints:
   2
2
2. The first statement prints: 2
   and the second throws a runtime exception
3. The first statement fails to compile
   and the second prints: 2
4. The first statement fails to compile
   and the second throws a runtime exception

Stringy Conversions

class A { implicit val stringToInt = (_: String).toInt println("4" - 2) } class B { implicit val stringToInt: String => Int = _.toInt println("4" - 2) } new A() new B()

Answer #2:
The first statement prints: 2
and the second throws a runtime exception

Since type of stringToInt in B is known upfront,
it is selected to perform the conversion,
causing an infinite loop

Miles Sabin: "Don't do this!"

Size It Up

import collection.mutable def howManyItems(lunchbox: mutable.Set[String], itemToAdd: String): Int = (lunchbox + itemToAdd).size def howManyItemsRefac(lunchbox: mutable.Iterable[String], itemToAdd: String): Int = (lunchbox + itemToAdd).size val lunchbox = mutable.Set("chocolate", "juice", "sandwich") println(howManyItems(lunchbox, "apple")) println(howManyItemsRefac(lunchbox, "apple")) println(lunchbox.size)

Size It Up

import collection.mutable def howManyItems(lunchbox: mutable.Set[String], itemToAdd: String): Int = (lunchbox + itemToAdd).size def howManyItemsRefac(lunchbox: mutable.Iterable[String], itemToAdd: String): Int = (lunchbox + itemToAdd).size val lunchbox = mutable.Set("chocolate", "juice", "sandwich") println(howManyItems(lunchbox, "apple")) println(howManyItemsRefac(lunchbox, "apple")) println(lunchbox.size)

Size It Up

import collection.mutable def howManyItems(lunchbox: mutable.Set[String], itemToAdd: String): Int = (lunchbox + itemToAdd).size def howManyItemsRefac(lunchbox: mutable.Iterable[String], itemToAdd: String): Int = (lunchbox + itemToAdd).size val lunchbox = mutable.Set("chocolate", "juice", "sandwich") println(howManyItems(lunchbox, "apple")) println(howManyItemsRefac(lunchbox, "apple")) println(lunchbox.size)

Answer #4:
4
36
3


+ is defined for Set, but not for Iterable

So, the second + represents String concatentation
and this applies because of the existence of the evil any2stringadd

One, Two, Skip a Few

val oneTwo = Seq(1, 2).permutations if (oneTwo.length > 0) { println("Permutations of 1 and 2:") oneTwo foreach println } val threeFour = Seq(3, 4).permutations if (!threeFour.isEmpty) { println("Permutations of 3 and 4:") threeFour foreach println }

One, Two, Skip a Few

val oneTwo = Seq(1, 2).permutations if (oneTwo.length > 0) { println("Permutations of 1 and 2:") oneTwo foreach println } val threeFour = Seq(3, 4).permutations if (!threeFour.isEmpty) { println("Permutations of 3 and 4:") threeFour foreach println }

One, Two, Skip a Few

val oneTwo = Seq(1, 2).permutations if (oneTwo.length > 0) { println("Permutations of 1 and 2:") oneTwo foreach println } val threeFour = Seq(3, 4).permutations if (!threeFour.isEmpty) { println("Permutations of 3 and 4:") threeFour foreach println }

Answer #4:
Permutations of 1 and 2:
Permutations of 3 and 4:
List(3, 4)
List(4, 3)

permutations returns an Iterator
which is mutable

Never call any methods except next and hasNext on an iterator
after calling a method on it.

A Listful of Dollars

type Dollar = Int final val Dollar: Dollar = 1 val a: List[Dollar] = List(1, 2, 3) println(a map { a: Int => Dollar }) println(a.map(a: Int => Dollar))

A Listful of Dollars

type Dollar = Int final val Dollar: Dollar = 1 val a: List[Dollar] = List(1, 2, 3) println(a map { a: Int => Dollar }) println(a.map(a: Int => Dollar))

A Listful of Dollars

type Dollar = Int final val Dollar: Dollar = 1 val a: List[Dollar] = List(1, 2, 3) println(a map { a: Int => Dollar }) println(a.map(a: Int => Dollar))

Answer #2:
The first one prints: List(1, 1, 1)
and the second one throws a runtime exception

{ a: Int => Dollar } is parsed as:
{ (a: Int) => Dollar }

(a: Int => Dollar) is parsed as:
(a: (Int => Dollar)}

List is a Function1 and { ... } can enclose a ResultExpr,
whereas ( ... ) can only enclose an Expr. A ResultExpr, but not an Expr, can include a type specification for its argument.

The Devil Is in the Defaults

import collection.mutable.Queue val goodies: Map[String, Queue[String]] = Map().withDefault(_ => Queue("Keep looking.")) val baddies: Map[String, Queue[String]] = Map().withDefaultValue(Queue("Nothing here.")) println(goodies("kitchen").dequeue) println(baddies("in attic").dequeue) println(goodies("dining room").dequeue) println(baddies("under bed").dequeue)

The Devil Is in the Defaults

import collection.mutable.Queue val goodies: Map[String, Queue[String]] = Map().withDefault(_ => Queue("Keep looking.")) val baddies: Map[String, Queue[String]] = Map().withDefaultValue(Queue("Nothing here.")) println(goodies("kitchen").dequeue) println(baddies("in attic").dequeue) println(goodies("dining room").dequeue) println(baddies("under bed").dequeue)

The Devil Is in the Defaults

import collection.mutable.Queue val goodies: Map[String, Queue[String]] = Map().withDefault(_ => Queue("Keep looking.")) val baddies: Map[String, Queue[String]] = Map().withDefaultValue(Queue("Nothing here.")) println(goodies("kitchen").dequeue) println(baddies("in attic").dequeue) println(goodies("dining room").dequeue) println(baddies("under bed").dequeue)

Answer #1:
First, second & third print:
Keep looking.
Nothing here.
Keep looking.
and the forth throws a runtime exception

The value provided for withDefaultValue
is shared across all map entries.

Return to Me!

def value: Int = { def one(x: Int): Int = { return x; 1 } val two = (x: Int) => { return x; 2 } 1 + one(2) + two(3) } println(value)

Return to Me!

def value: Int = { def one(x: Int): Int = { return x; 1 } val two = (x: Int) => { return x; 2 } 1 + one(2) + two(3) } println(value)

1. Prints: 3
2. Prints: 4
3. Prints: 6
4. Fails with a compilation error: unreachable code

Return to Me!

def value: Int = { def one(x: Int): Int = { return x; 1 } val two = (x: Int) => { return x; 2 } 1 + one(2) + two(3) } println(value)

Answer #1:
Prints 3

return statement returns from enclosing def
and function values do not qualify as an enclosing function.

Set the Record Straight

val numbers = List("1", "2").toSet() + "3" println(numbers)

Set the Record Straight

val numbers = List("1", "2").toSet() + "3" println(numbers)

1. Prints: Set(1, 2, 3)
2. Fails with a compilation error
3. Prints: false3
4. Prints: 123

Set the Record Straight

val numbers = List("1", "2").toSet() + "3" println(numbers)

Answer #3:
Prints: false3

toSet method is parameterless

empty argument list is adapted to (): Unit
and is passed to the apply method on the resulting set,
which tests for inclusion of an item in the set.
This is likely to "go away" in future versions.

What's in a Name?

class C { def sum(x: Int = 1, y: Int = 2): Int = x + y } class D extends C { override def sum(y: Int = 3, x: Int = 4): Int = super.sum(x, y) } val d: D = new D val c: C = d println(c.sum(x = 0)) println(d.sum(x = 0))

What's in a Name?

class C { def sum(x: Int = 1, y: Int = 2): Int = x + y } class D extends C { override def sum(y: Int = 3, x: Int = 4): Int = super.sum(x, y) } val d: D = new D val c: C = d println(c.sum(x = 0)) println(d.sum(x = 0))

What's in a Name?

class C { def sum(x: Int = 1, y: Int = 2): Int = x + y } class D extends C { override def sum(y: Int = 3, x: Int = 4): Int = super.sum(x, y) } val d: D = new D val c: C = d println(c.sum(x = 0)) println(d.sum(x = 0))

Answer #3:
Prints:
4
3



As Josh Suereth summarizes:
Names are static; values are runtime.

Thanks!

Any questions?