Send + More = Money – ScalaSyd – July 2015

SendMoreMoneySpec.scala

import org.specs2._
import scalaz._, Scalaz._

class SendMoreMoneySpec extends Specification { def is = s2"""
 Using the List Monad
   List(1, 2)                    $tl1
   List(1, 2) if a != b          $tl2
   List(1, 2, 3)                 $tl3
   Map('a' -> a, 'b' -> b)       $tl4
   s.eval(List(1, 2))            $ts1
   s.eval(List(1, 2, 3))         $tsl1
   g.eval(List(1, 2, 3))         $tg1
                                 """

  val l1 = for {
    a <- List(1, 2)
    b <- List(1, 2)
  } yield (a, b)

  val tl1 = l1 ==== List((1, 1), (1, 2), (2, 1), (2, 2))

  // ---------------------------------------------------------------------------

  val l2 = for {
    a <- List(1, 2)
    b <- List(1, 2) if a != b
  } yield (a, b)

  val tl2 = l2 ==== List((1, 2), (2, 1))

  // ---------------------------------------------------------------------------

  val l3 = for {
    a <- List(1, 2, 3)
    b <- List(1, 2, 3)
  } yield (a, b)

  val tl3 = l3 ==== List((1,1), (1,2), (1,3), (2,1), (2,2), (2,3), (3,1), (3,2), (3,3))

  // ---------------------------------------------------------------------------

  val l4 = for {
    a <- List(1, 2)
    b <- List(1, 2) if a != b
  } yield Map('a' -> a, 'b' -> b)

  val tl4 = l4 ==== List(Map('a' -> 1, 'b' -> 2), Map('a' -> 2, 'b' -> 1))

  // ---------------------------------------------------------------------------

  val l5 = for {
    a <- List(1, 2, 3)
    b <- List(1, 2, 3) if a != b
    c <- List(1, 2, 3) if a != c && b != c
  } yield (a, b, c)

  val tl5 = l5 ==== List((1,2,3), (1,3,2), (2,1,3), (2,3,1), (3,1,2), (3,2,1))

  // ---------------------------------------------------------------------------

  import scalaz.State

  def selectS(xs: List[Int]): (List[Int], Int) = (xs.tail, xs.head)

  val s = for {
    a <- State(selectS)
    b <- State(selectS)
  } yield Map('a' -> a, 'b' -> b)

  val ts1 = s.eval(List(1, 2)) ==== Map('a' -> 1, 'b' -> 2)

  val ts2 = s.eval(List(1, 2, 3)) ==== Map('a' -> 1, 'b' -> 2)

  // ---------------------------------------------------------------------------

  import scalaz.StateT
  import scalaz.std.list._

  def selectSL[A](xs: List[A]): List[(List[A], A)] =
    xs map { x => (xs.filterNot(_ == x), x) }

  val sl = for {
    a <- StateT(selectSL[Int])
    b <- StateT(selectSL[Int])
  } yield Map('a' -> a, 'b' -> b)

  val exp: List[Map[Char, Int]] = List(Map('a' -> 1, 'b' -> 2), Map('a' -> 2, 'b' -> 1))

  val tsl1 = sl.eval(List(1, 2)) ==== List(Map('a' -> 1, 'b' -> 2), Map('a' -> 2, 'b' -> 1))

  // ---------------------------------------------------------------------------

  type StateL[A] = StateT[List, List[Int], A]

  val g = for {
    a <- StateT(selectSL[Int])
    b <- StateT(selectSL[Int])
    c <- StateT(selectSL[Int])
    _ <- (a + b == c).guard[StateL](())
    _ <- (a > b).prevent[StateL](())
  } yield Map('a' -> a, 'b' -> b, 'c' -> c)

  val tg1 = g.eval(1 |-> 3) ==== List(Map('a' -> 1, 'b' -> 2, 'c' -> 3))

}