为什么Scala函数接受函数参数的大小写类类型？

Question

Consider the code block.

class Evaluation {
  def evaluate= {
    println("Charlie...")
  }
}
case class Dept(name:String) extends Evaluation

def depEval(name:String,f:(String) => Evaluation) ={
  println(name)
  f(name).evaluate
}
depEval("abc", Dept)

Why does Dept can be passed as a Funtion1 type? Is it that, Scala does not checks for the type before resolving the arguments.

Consider other snippet

def matchingCase(f: String => Evaluation)= {
  println(f.toString())
  f match {
    case Dept => println("function type matched")
  }
}

matchingCase((x: String)=> Dept(x))

Whereas in the above code scala gives a match error, as expected.

Answer 1

The companion object of a case class extends FunctionX[T1, T2, <CaseClass>] so that you can use it to construct instances of the case class.

So, for example, if you have a case class

case class Foo(i: Int, s: String)

the compiler will generate a companion object

object Foo extends (Int, String) ⇒ Foo with Product2[Int, String] {
  def apply(i: Int, s: String) = new Foo(i, s)

  // and some other stuff such as equals, hashCode, copy
}

This allows you to construct an instance of Foo like this:

Foo(42, "Hello")

instead of

new Foo(42, "Hello")

So, to conclude: the reason, why you can pass the Dept companion object as a function is because it is a function.

Answer 2

Dept is not a "case class type", it's the companion object for the case class. It extends Function1 (and has .apply method, that's part of the interface), so you can use it wherever a function is expected.

The second snipped fails because { x => Dept(x) } is it's own anonymous function, not Dept object.

Answer 3

It has a lot of sense, because if you just give as paramenter Dept it will behave ass a function that takes a String as parameter, and return an instance of the class Dept, meaning a Dept object (It is like give the new or apply function as parameter), look that the following code is valid also:

val c: Dept.type = Dept

val dep = c("Name1")

dep

res1: Dept = Dept(Name1)

dep.evaluate

Charlie... res2: Unit = ()