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package scala
abstract class MatchingStrategy[M[+x]] {
def zero: M[Nothing]
def one[T](x: T): M[T]
def guard[T](cond: Boolean, then: => T): M[T] // = if(cond) one(then) else zero
def altFlatMap[T, U](f: T => M[U])(a: M[U], b: M[T]): M[U] // = a orElse b.flatMap(f) -- can't easily&efficiently express M[T] should have flatMap and orElse
def runOrElse[T, U](x: T)(f: T => M[U]): U
def isSuccess[T, U](x: T)(f: T => M[U]): Boolean
// find the first alternative to successfully flatMap f
// to avoid code explosion due to alternatives
def or[T, U](f: T => M[U], alts: M[T]*) = (alts foldLeft (zero: M[U]))(altFlatMap(f))
def caseResult[T](x: T): M[T] = one(x) // used as a marker to distinguish the RHS of a case (case pat => RHS) and intermediate successes
// when deriving a partial function from a pattern match, we need to
// distinguish the RHS of a case, which should not be evaluated when computing isDefinedAt,
// from an intermediate result (which must be computed)
}
object MatchingStrategy {
implicit object OptionMatchingStrategy extends MatchingStrategy[Option] {
type M[+x] = Option[x]
@inline def guard[T](cond: Boolean, then: => T): M[T] = if(cond) Some(then) else None
@inline def zero: M[Nothing] = None
@inline def one[T](x: T): M[T] = Some(x)
@inline def altFlatMap[T, U](f: T => M[U])(a: M[U], b: M[T]): M[U] = a orElse b.flatMap(f)
@inline def runOrElse[T, U](x: T)(f: T => M[U]): U = f(x) getOrElse (throw new MatchError(x))
@inline def isSuccess[T, U](x: T)(f: T => M[U]): Boolean = !f(x).isEmpty
}
}
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