What Can Scala Puzzlers Teach Us?

A look at some of the most common puzzlers faced by Scala beginners.

1. What Can Scala Puzzlers Teach Us? Daniel C. Sobral
2. Disclaimer #1: "Puzzlers" • Java Puzzlers (the book/presentations): ◦ Weird behavior in Java that is difficult to predict or explain • This presentation: ◦ Weird behavior in Scala that confounds beginners ▪ (and sometimes experienced programmers) ◦ FAQs, not Fun
3. I can't speak for any of the people who contributed to create and turn Scala into what it is. Furthermore, if you learn anything from this presentation, learn that a perfect understanding of Scala is an unlikely achievement... Disclaimer #2: don't trust me.
4. Symbols, operators and punctuation • What does it mean? • Where can I find information about it? • WTF?
5. The symbol puzzler Source: Stack Overflow
6. The Origin of the Symbols • Punctuation elements of the language ◦ @ # ( ) [ ] { } , . ; : ` ' " • Keywords, reserved symbols and XML ◦ <- => <: >: <% // /* */ _* <? <! • Normal definitions from libraries ◦ <:< =:= ☆ ★ η :/ • Underscore ◦ All of the above!
7. Symbols may be part of the language or come from external libraries; it can be difficult to tell where they begin and end; they are difficult to search for on the Internet, non-mnemonic and unpronounceable.
8. Then why? def f(xs: Array[Int], a: Int, b: Int) = for (i <- xs.indices) xs(i) = a * i + b
9. So that libraries can extend the language seamlessly def f(xs: M[T], a: T, b: T) = for (i <- xs.indices) xs(i) = a * i + b For M and T library-types that support these operations
10. Which still doesn't really justify unicode...
11. Advice regarding Symbols • Learn the language: ◦ What symbols are "built-in" ◦ The syntax rules • If exploring code using unknown libraries, ◦ Use an IDE to explore it harsh, indeed...
12. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = { new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] { def id[A] = ☆(_ η) def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g } } Let's try to figure this one out
13. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = { new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] { def id[A] = ☆(_ η) def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g } } Split Scala Syntax from Identifiers
14. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = { new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] { def id[A] = ☆(_ η) def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g } } Identify what we are defining
15. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = { new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] { def id[A] = ☆(_ η) def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g } } Where we are using our definitions
16. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = { new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] { def id[A] = ☆(_ η) def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g } } What's left are outside identifiers
17. The Underscore • What does it mean? • Why does it work here but not here? • WTF? • It's everywhere! It's the borg operator!
18. One underscore puzzler Source: Stack overflow
19. The meanings of underscore Source: myself, by way of Stack Overflow
20. The underscore is Scala's wildcard; it means "something" or "anything" and even "nothing". However, it's semantics can be widely different, even in similar contexts.
21. Why? • ??? • Picking a different symbol for each case would stretch ASCII • Picking keywords is not in the spirit of Scala • They are somewhat intuitive ◦ except when they bite you
22. Advice regarding underscore • Intuition does go a long way • But, again, learn the language • And, in particular, know the difference between these: ◦ f _ ◦ f(_, y) ◦ f(x + _, y)
23. Eta, partial function application and placeholders • f _ ◦ an eta expansion: turn method f into an anonymous function. • f(_, y) ◦ a partial function application: creates an anonymous function from method f, whose parameters are the ones wholly replaced by underscores. • f(x + _, y) ◦ a placeholder: creates an anonymous function from the expression "x + _", whose parameters are the underscores (and that function gets passed to f).
24. _ always picks the tightest non-degenerate scope it can Seth Tisue
25. Erasure • Where are my types? • How do I get them back? • WTF?
26. Some things that don't work trait X { def overload(f: Int => Int) def overload(f: String => String) }
27. Some things that don't work def f[T](list: List[T]) = list match { case _: List[Int] => "Ints" case _: List[String] => "Strings" case _ => "???" }
28. Some things that don't work class C[T] { def f(obj: Any) = obj match { case _: T => "Ours" case _ => "Not Ours" } }
29. Why? • JVM does not support "type parameters" • If Scala added metadata to "eliminate" erasure, the interaction gap between it and Java would be much greater ◦ Though things like implicits and default parameters already do it, to some extent ◦ At least, implicits and defaults are easy to avoid
30. Some things that don't work trait X { def overload(f: Function1[Int, Int]) def overload(f: Function1[String, String]) }
31. Erasure at work trait X { def overload(f: Function1) def overload(f: Function1) }
32. Erasure at work def f(list: List) = list match { case _: List => "Ints" case _: List => "Strings" case _ => "???" }
33. Erasure at work class C { def f(obj: Any) = obj match { case _: Any => "Ours" case _ => "Not Ours" } }
34. A partial solution class C[T : ClassTag] { def f(obj: Any) = obj match { case _: T => "Ours" case _ => "Not Ours" } } though it will still ignore T's type parameters
35. Initialization Order trait A { val a: Int val b = a * 2 } class B extends A { val a = 5 println(b) } new B
36. All val’s are initialized in the order they are found.
37. Initialization Order trait A { val foo: Int val bar = 10 println("In A: foo: " + foo + ", bar: " + bar) } class B extends A { val foo: Int = 25 println("In B: foo: " + foo + ", bar: " + bar) } class C extends B { override val bar = 99 println("In C: foo: " + foo + ", bar: " + bar) } new C Source: Paul Phillips scala-faq by way of scala puzzlers
38. Initialization Order In A: foo: 0, bar: 0 In B: foo: 25, bar: 0 In C: foo: 25, bar: 99
39. Initialization Order trait A { val foo: Int val bar = 10 println("In A: foo: " + foo + ", bar: " + bar) } class B extends A { val foo: Int = 25 println("In B: foo: " + foo + ", bar: " + bar) } class C extends B { override val bar = 99 println("In C: foo: " + foo + ", bar: " + bar) } new C
40. A val is initialized only once.
41. Initialization Order trait A { val x = 5 } trait B { val x: Int println(x * 2) } trait C extends A with B trait D extends B class E extends D with C { println(x * 2) } class F extends C with D { println(x * 2) } new E new F
42. Initialization Order 0 10 10 10
43. Initialization Order trait A { val x = 5 } trait B { val x: Int println(x * 2) } trait C extends A with B trait D extends B class E extends B with D with A with B with C { println(x * 2) } class F extends A with B with C with B with D { println(x * 2) } new E new F
44. Trait linearization...
45. Advice regarding initialization order • Learn the _________
46. Other things to look out for • Pattern matching on val's and for's • All sorts of things implicit • Syntactic sugars, auto-tupling and the like • The return keyword
47. So, what did I learn? It seems to me that there's a wide range of Scala features that depend on the user having deep knowledge of the language when something goes wrong. "What's going on?" seems to be much more common than "How do I do this?"
48. As for real Puzzlers... • Site: ◦ http://scalapuzzlers.com/ • Book: ◦ http://www.artima.com/shop/scala_puzzlers