conjugations.scala

// these are the data types
trait Tense
case object Past extends Tense
case object Present extends Tense
case object Future extends Tense

trait Subject {
  protected val usage: Map[Tense, String]
  def apply(tense: Tense): String = usage(tense)
}
case object Je extends Subject {
  protected val usage: Map[Tense, String] = Map(Past -> "je", Present -> "j'ai", Future -> "je")
}
case object Ils extends Subject {
  protected val usage: Map[Tense, String] = Map.empty
}

// it seemed reasonable for verbs to be able to stem themsevles
// as (as far as I know) the stemming is the first thing done before
// applying any of the conjugations 
case class Verb(given: String, tense: Tense, subject: Subject){
  val suffix: Option[String] = List("e", "es", "ons", "er").find(given.endsWith(_))
  def asStem = this.copy(given = suffix.map(given.stripSuffix(_)).getOrElse(given))
  def inFrench: String = "%s %s".format(subject(tense), given)
}

val say: Verb => Verb = { verb =>
  def suffix(letters: String) = verb.copy(given = verb.asStem.given + letters)
  (verb.suffix, verb.subject) match {
    case (Some("er"), Je) => suffix("e")
    case (Some("er"), Ils) => suffix("ez")
    case _ => verb // FIXME: pass through for all other cases 
  }
}

println {
  // je regarde
  say(Verb("regarder", Past, Je)) inFrench
  
  // j'ai regarde
  say(Verb("regarder", Present, Je)) inFrench
  
}