How can (1 +) be ever a function?

Question

I am new to Scala, and trying to understand the following codes (derived from an example in the Beginning Scala book)

scala> def w42(f: Int => Int) = f(42)  //(A)
w42: (f: Int => Int)Int

scala> w42 (1 +)      //(B)
res120: Int = 43

I do not understand how "1 +" at point (B) is consider as a function (take 1 Int parameter, and return an Int) that satisfies the w42 definition at point (A)?

Would you mind please explain or point me to some documents that have the answer?

Answer 1

Simple. In Scala 1 + 2 is just a syntax sugar over 1.+(2). This means Int has a method named + that accepts Int:

final class Int extends AnyVal {
  def +(x: Int): Int = //...
  //...
}

This is why you can use 1 + as if it was a function. Example with less unexpected method naming:

scala> def s42(f: String => String) = f("42")
s42: (f: String => String)String

scala> s42("abc".concat)
res0: String = abc42

BTW Technically speaking, eta-expansion is also involved to convert method to a function.