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I'm studying the scala red book published by Manning

The List implementation is a case class:

case class Cons[+A](head: A, tail: List[A]) extends List[A]

foldRight is defined as:

def foldRight[A, B](as: List[A], z: B)(f: (A, B) => B): B

and implemented as 

  def foldRight[A, B](as: List[A], z: B)(f: (A, B) => B): B = { // Utility functions
    as match {
      case Nil => z
      case Cons(h, t) => f(h, foldRight(t, z)(f))
    }
  }

To append one List to another, my solution was:

  def append[A](l1: List[A], l2: List[A]): List[A] 
      = foldRight(l1, l2)((l1head,l2)=>Cons(l1head, l2))

the answer key, however, shows the following code:

  def append[A](l1: List[A], l2: List[A]): List[A] = foldRight(l1, l2)(Cons(_, _))

Question: How does (Cons(_, _))conform to the method signature of f: (A, B) => B

Jake
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1 Answers1

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Both append() examples are exactly the same. The 1st underscore is shorthand for "the 1st passed-in parameter" and the 2nd underscore is shorthand for "the 2nd passed-in parameter." It's just a handy way of referencing them, and passing them on to the Cons constructor, without the bother of giving each a name.

See also the various answers to this question.

jwvh
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