_必須追蹤[咖喱]方法

Question

我正在map3從Functional Programming in Scala：

// Exercise 3: The apply method is useful for implementing map3, map4, and so on 
    // and the pattern is straightforward. Implement map3 and map4 using only 
    // unit, apply, and the curried available on functions. 
    def map3[A,B,C,D](fa: F[A], 
        fb: F[B], 
        fc: F[C])(f: (A, B, C) => D): F[D] = { 
    def foo: (A => B => C => D) = (f _).curried // compile-time error 
    def fooF: F[A => B => C => D] = unit(foo) 
    val x: F[B => C => D] = apply(fooF)(fa) 
    val y: F[C => D] = apply(x)(fb) 
    val z: F[D] = apply(y)(fc) 
    z 
    } 

一（1）編譯時以上，注意的行發生錯誤：

[error] C:\...\Applicative.scala: _ must follow method; cannot follow f.type 
[error]  def foo: (A => B => C => D) = (f _).curried 
[error] 

          ^

我能夠通過這篇文章成功獲得curried版本的功能 - Using FunctionX#curried。

但是，我不明白上面的編譯時錯誤。

Answer 1

錯誤是因爲f這裏是一個函數，而不是一個方法; f _語法是將方法轉換爲函數，在此不需要。簡單地寫def foo: A => B => C => D = f.curried。