实现折叠[列表[A]]

Question

我正在进行Functional Programming in Scala练习以实现List[A]以下特质。

trait Foldable[F[_]] { 
    def foldRight[A, B](as: F[A])(f: (A, B) => B): B 
    def foldLeft[A, B](as: F[A])(f: (B, A) => B): B 
    def foldMap[A, B](as: F[A])(f: A => B)(mb: Monoid[B]): B 
    def concatenate[A](as: F[A])(m: Monoid[A]): A = foldLeft(as)(m.zero)(m.op) 
} 

在我尝试实现foldLeft，我怎么可以指定initial值，如果有没有在签名？

trait Foldable[List[_]] { 
    def foldLeft[A,B](as: List[A])(f: (A, B) => B): B = { 
     go(bs: List[A], acc: B): B = bs match { 
      case x :: xs => go(xs, f(x, acc)) 
      case Nil => acc 
     } 
    go(as, ???) // No start value in the signature? And no Monoid for m.zero 
    } 
} 

Answer 1

这是本书中的错误。看看github上源，你会看到一个零方法PARAM：

https://github.com/pchiusano/fpinscala/blob/master/exercises/src/main/scala/fpinscala/monoids/Monoid.scala#L84

trait Foldable[F[_]] { 
    import Monoid._ 

    def foldRight[A, B](as: F[A])(z: B)(f: (A, B) => B): B = 
    sys.error("todo") 

    def foldLeft[A, B](as: F[A])(z: B)(f: (B, A) => B): B = 
    sys.error("todo") 

    def foldMap[A, B](as: F[A])(f: A => B)(mb: Monoid[B]): B = 
    sys.error("todo") 

    def concatenate[A](as: F[A])(m: Monoid[A]): A = 
    sys.error("todo") 

    def toList[A](as: F[A]): List[A] = 
    sys.error("todo") 
}