Changing Set to Seq (#42)

* Changing Set to Seq

`Set`'s `headOption` is not well defined, while `Seq`'s `headOption` is.

* Fixed remaining Sets

src/main/scala/shapeless/PolyExercises.scala

@@ -18,30 +18,30 @@ import poly.{~>}
 */
object PolyExercises extends FlatSpec with Matchers with org.scalaexercises.definitions.Section {

  object choose extends (Set ~> Option) {
    def apply[T](s: Set[T]) = s.headOption
  object choose extends (Seq ~> Option) {
    def apply[T](s: Seq[T]) = s.headOption
  }

  /** `choose` is a function from Sets to Options with no type specific cases
  /** `choose` is a function from Seqs to Options with no type specific cases
   * {{{
   * object choose extends (Set ~> Option) {
   * def apply[T](s : Set[T]) = s.headOption
   * object choose extends (Seq ~> Option) {
   * def apply[T](s : Seq[T]) = s.headOption
   * }
   * }}}
   */
  def exerciseChoose(res0: Option[Int], res1: Option[Char]) = {
    import shapeless.poly._
    // choose is a function from Sets to Options with no type specific cases
    // choose is a function from Seqs to Options with no type specific cases

    choose(Set(1, 2, 3)) should be(res0)
    choose(Set('a', 'b', 'c')) should be(res1)
    choose(Seq(1, 2, 3)) should be(res0)
    choose(Seq('a', 'b', 'c')) should be(res1)
  }

  /** Being polymorphic, they may be passed as arguments to functions or methods and then applied to values of different types
   * within those functions
   */
  def exercisePairApply(res0: Option[Int], res1: Option[Char]) = {
    def pairApply(f: Set ~> Option) = (f(Set(1, 2, 3)), f(Set('a', 'b', 'c')))
    def pairApply(f: Seq ~> Option) = (f(Seq(1, 2, 3)), f(Seq('a', 'b', 'c')))

    pairApply(choose) should be(res0, res1)
  }
@@ -51,7 +51,7 @@ object PolyExercises extends FlatSpec with Matchers with org.scalaexercises.defi
   * mapped across an ordinary Scala List
   */
  def exerciseMonomorphicChoose(res0: Option[Int], res1: Option[Int]) =
    (List(Set(1, 3, 5), Set(2, 4, 6)) map choose) should be(List(res0, res1))
    (List(Seq(1, 3, 5), Seq(2, 4, 6)) map choose) should be(List(res0, res1))

  /** However, they are [[http://www.chuusai.com/2012/05/10/shapeless-polymorphic-function-values-2/ more general than natural transformations]] and are able to capture type-specific cases
   * which, as we'll see below, makes them ideal for generic programming,