MonadPlus and the guard function 

Scala’s for notation allows filtering:

scala> for {
         x <- 1 |-> 50 if x.shows contains '7'
       } yield x
res40: List[Int] = List(7, 17, 27, 37, 47)

LYAHFGG:

The MonadPlus type class is for monads that can also act as monoids.

Here’s the typeclass contract for MonadPlus:

trait MonadPlus[F[_]] extends Monad[F] with ApplicativePlus[F] { self =>
  ...
}

Plus, PlusEmpty, and ApplicativePlus 

It extends ApplicativePlus:

trait ApplicativePlus[F[_]] extends Applicative[F] with PlusEmpty[F] { self =>
  ...
}

And that extends PlusEmpty:

trait PlusEmpty[F[_]] extends Plus[F] { self =>
  ////
  def empty[A]: F[A]
}

And that extends Plus:

trait Plus[F[_]]  { self =>
  def plus[A](a: F[A], b: => F[A]): F[A]
}

Similar to Semigroup[A] and Monoid[A], Plus[F[_]] and PlusEmpty[F[_]] requires their instances to implement plus and empty, but at the type constructor ( F[_]) level.

Plus introduces <+> operator to append two containers:

scala> List(1, 2, 3) <+> List(4, 5, 6)
res43: List[Int] = List(1, 2, 3, 4, 5, 6)

MonadPlus again 

MonadPlus introduces filter operation.

scala> (1 |-> 50) filter { x => x.shows contains '7' }
res46: List[Int] = List(7, 17, 27, 37, 47)