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20 Intermediate Scala Exercises

I will be publishing answers to these exercises as well as the Revised Scala Exercises in the near future. In the meantime, see if you can tackle the exercises below. In almost all cases, the type signature is enough to derive what the implementation should be. In all other cases, the solution is as difficult to derive anyway, so consider it an equivalent achievement.

The source file below compiles, but is missing certain function implementations (where you see error("todo")). Enjoy and if you have questions, please feel free to post them

trait PartialType[T[_, _], A] {
  type Apply[B] = T[A, B]
  type Flip[B] = T[B, A]
}
 
trait Fluffy[F[_]] {
  def furry[A, B](f: A => B, fa: F[A]): F[B]
}
 
object Fluffy {
  // Exercise 1
  // Relative Difficulty: 1
  def ListFluffy: Fluffy[List] = error("todo")
 
  // Exercise 2
  // Relative Difficulty: 1
  def OptionFluffy: Fluffy[Option] = error("todo")
 
  // Exercise 3
  // Relative Difficulty: 1
  def StreamFluffy: Fluffy[Stream] = error("todo")
 
  // Exercise 4
  // Relative Difficulty: 1
  def ArrayFluffy: Fluffy[Array] = error("todo")
 
  // Exercise 5
  // Relative Difficulty: 5
  def Function1Fluffy[X]: Fluffy[PartialType[Function1, X]#Apply] =
    error("todo")
 
  // Exercise 6
  // Relative Difficulty: 6
  def EitherLeftFluffy[X]: Fluffy[PartialType[Either.LeftProjection, X]#Flip] =
    error("todo")
 
  // Exercise 7
  // Relative Difficulty: 4
  def EitherRightFluffy[X]: Fluffy[PartialType[Either.RightProjection, X]#Apply] =
    error("todo")
}
 
trait Misty[M[_]] extends Fluffy[M] {
  def banana[A, B](f: A => M[B], ma: M[A]): M[B]
 
  def unicorn[A](a: A): M[A]
 
  // Exercise 8
  // Relative Difficulty: 3
  // (use banana and/or unicorn)
  def furry[A, B](f: A => B, ma: M[A]): M[B] = error("todo")
}
 
object Misty {
  // Exercise 9
  // Relative Difficulty: 2
  def ListMisty: Misty[List] = error("todo")
 
  // Exercise 10
  // Relative Difficulty: 2
  def OptionMisty: Misty[Option] = error("todo")
 
  // Exercise 11
  // Relative Difficulty: 2
  def StreamMisty: Misty[Stream] = error("todo")
 
  // Exercise 12
  // Relative Difficulty: 2
  def ArrayMisty: Misty[Array] = error("todo")
 
  // Exercise 13
  // Relative Difficulty: 6
  def Function1Misty[X]: Misty[PartialType[Function1, X]#Apply] =
    error("todo")
 
  // Exercise 14
  // Relative Difficulty: 7
  def EitherLeftMisty[X]: Misty[PartialType[Either.LeftProjection, X]#Flip] =
    error("todo")
 
  // Exercise 15
  // Relative Difficulty: 5
  def EitherRightMisty[X]: Misty[PartialType[Either.RightProjection, X]#Apply] =
    error("todo")
 
  // Exercise 16
  // Relative Difficulty: 3
  def jellybean[M[_], A](ma: M[M[A]], m: Misty[M]): M[A] = error("todo")
 
  // Exercise 17
  // Relative Difficulty: 6
  def apple[M[_], A, B](ma: M[A], mf: M[A => B], m: Misty[M]): M[B] =
    error("todo")
 
  // Exercise 18
  // Relative Difficulty: 6
  def moppy[M[_], A, B](as: List[A], f: A => M[B], m: Misty[M]): M[List[B]] =
    error("todo")
}
 
object AdvancedFun {
  case class State[S, A](f: S => (S, A))
 
  // Exercise 19
  // Relative Difficulty: 9
  def StateFluffy[S]: Fluffy[PartialType[State, S]#Apply] = error("todo")
 
  // Exercise 20
  // Relative Difficulty: 10
  def StateMisty[S]: Misty[PartialType[State, S]#Apply] = error("todo")
}

This entry was posted on Wednesday, September 10th, 2008 at 1:36 pm and is filed under Programming. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

13 Responses to “20 Intermediate Scala Exercises”

German Buela Says:
September 12th, 2008 at 1:32 am
Boy, do you make my brain hurt.
Tony Morris Says:
September 12th, 2008 at 6:31 am
Good exercise

I am considering writing some “tests” for these functions to make their intention clear. Would this help? It will just add to the verbosity of the code though.
German Buela Says:
September 12th, 2008 at 1:46 pm
Please write them. I’m getting my implementations to compile but I’m not quite sure they all do the right thing.
Jeff Says:
September 12th, 2008 at 6:43 pm
These exercises are great. Thanks a lot Tony. Currently I’m stumped on test 13 and tests would help give me confidence that I am on the right track so far.
Tony Morris Says:
September 12th, 2008 at 9:35 pm
G’day Jeff, I’m working on some tests. Expect them soon!
German Buela Says:
September 15th, 2008 at 11:49 pm
I’m stumped on #13 as well, but I doubt the tests will help me go further :(… at least I’ll be able to validate the first twelve.
Tony Morris Says:
September 16th, 2008 at 3:35 pm
Hi Jeff and German,
Sorry I have just had got out of hospital following an ankle reconstruction. Can you at least get #13 to type check? e.g. Misty[PartialType[Function1, X]#Apply] { * have you written these type signatures? */}.

I’m not sure I’ll be able to get around to providing tests (I am currently high on morphine and things will be a bit difficult for a while :), but I can paraphrase them:

1. All instances of Fluffy must satisfy two rules:
1.1 For any value of x, furry(identity, x) == x
1.2. For any value x and any two functions f and g, furry(t => f(g(t)), x) == furry(f, furry(g, x))

2. All instances of Misty must satisfy three rules:
2.1 For any value x and any function f, banana(f, unicorn(x)) == f(x)
2.2 For any value x, banana(t => unicorn(t), x) == x
2.3 For any value x and any two functions f and g, banana(g, banana(f, x)) == banana(t => banana(g, f(a)), x)

I hope jellybean, apple and moppy are good enough given by their type signatures (actually apple can be written two different ways, but if you find one of those ways the other is almost immediately obvious anyway).

If you have any more questions or need some hints, then please feel free to drop me a line.
Jeff Says:
September 17th, 2008 at 7:37 pm
Hi Tony

I’m very grateful that you are bothering to respond to these comments while convalescing. The information you provided is useful for convincing myself of the correctness of my answers but I don’t know how to prove them. I think I have finally figured out exercise 13; I have at least got it to type check. I’m trying to figure out apple now.

Cheers,
Jeff
Tony Morris Says:
September 17th, 2008 at 8:04 pm
No worries Jeff, I hope you can pull through with these exercises. I’m assuming you are unaware of what some of the euphemisms stand for in which case, there will be a couple of surprises once you are done!

If your intuition says that your code meets the properties that I gave, then I suspect you are right. It would be difficult and more convoluted to not meet these properties in many cases.

Let me know if you get stuck again since I’d really like to see you get through.
Klaus Says:
September 24th, 2008 at 4:07 am
Hi Tony,

I got all exercises to compile. Do you have time to have a look? Can I post them here?
Tony Morris Says:
September 24th, 2008 at 6:11 am
Sure Klaus, I’ll be posting some answers soon.
Klaus Says:
September 25th, 2008 at 3:29 am
Hi Tony,

here are my solutions - base64 encoded:

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Tony Morris Says:
December 28th, 2009 at 12:29 pm
For #13, if you have an answer that type-checks and doesn’t use any bottom values (e.g. non-termination, error, null), then you must have the answer. In other words, its type signature is once-inhabited. So it’s not possible to write tests for #13 in a way that is not redundant.

20 Intermediate Scala Exercises

13 Responses to “20 Intermediate Scala Exercises”

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