A base class to solve the Johnny Hates Math problem found at http://www.spoj.com/problems/ANARC07J/ (NB: this is to find all solutions to the problem, not to satisfy the SPOJ IO specification)

JHMSolver.scala

package jhm

abstract class JHMSolver(digits: String, total: Int, maxDigitsPerNumber: Int = 5)
{
  def solve(digits: String, total: Int, maxDigitsPerNumber: Int = 5): List[List[Int]]
  
  lazy val solutions = solve(digits, total, maxDigitsPerNumber)

  def getSolutionsAsString(): String = solutions match {
    case Nil => "IMPOSSIBLE"
    case slns => slns map (
      (solution: List[Int]) => s"${ solution mkString "+" }=$total"
    ) mkString "\n"
  }
}

object JHMSolver {
  def getDefaultSolver(digits: String, total: Int): JHMSolver 
  	= new SolverUsingFlatMap(digits, total, maxDigitsPerNumber = 5)
  
  def getSolutions(digits: String, total: Int): List[List[Int]] = {
    getDefaultSolver(digits, total).solutions
  }
  
  def getSolutionsAsString(digits: String, total: Int): String = {
    getDefaultSolver(digits, total).getSolutionsAsString
  }
}