MatchMonadInterface

Interface with user-defined match monad? if there's a __match in scope, we use this as the match strategy, assuming it conforms to MatchStrategy as defined below:

type Matcher[P[_], M[+_], A] = {
  def flatMap[B](f: P[A] => M[B]): M[B]
  def orElse[B >: A](alternative: => M[B]): M[B]
}

abstract class MatchStrategy[P[_], M[+_]] {
  // runs the matcher on the given input
  def runOrElse[T, U](in: P[T])(matcher: P[T] => M[U]): P[U]

  def zero: M[Nothing]
  def one[T](x: P[T]): M[T]
  def guard[T](cond: P[Boolean], then: => P[T]): M[T]
}

P and M are derived from one's signature (def one[T](x: P[T]): M[T])

if no __match is found, we assume the following implementation (and generate optimized code accordingly)

object __match extends MatchStrategy[({type Id[x] = x})#Id, Option] {
def zero = None
def one[T](x: T) = Some(x)
// NOTE: guard's return type must be of the shape M[T], where M is the monad in which the pattern match should be interpreted
def guard[T](cond: Boolean, then: => T): Option[T] = if(cond) Some(then) else None
def runOrElse[T, U](x: T)(f: T => Option[U]): U = f(x) getOrElse (throw new MatchError(x))
}