Why does Scala need parameterless in addition to zero-parameter methods?

Question

I understand the difference between zero-parameter and parameterless methods, but what I don\'t really understand is the language design choice that made parameterless metho

Answer 1

Besides the convention fact mentioned (side-effect versus non-side-effect), it helps with several cases:

Usefulness of having empty-paren

// short apply syntax

object A {
  def apply() = 33
}

object B {
  def apply   = 33
}

A()   // works
B()   // does not work

// using in place of a curried function

object C {
  def m()() = ()
}

val f: () => () => Unit = C.m

Usefulness of having no-paren

// val <=> def, var <=> two related defs

trait T { def a:   Int; def a_=(v: Int): Unit }
trait U { def a(): Int; def a_=(v: Int): Unit }

def tt(t: T): Unit = t.a += 1  // works
def tu(u: U): Unit = u.a += 1  // does not work

// avoiding clutter with apply the other way round

object D {
  def a   = Vector(1, 2, 3)
  def b() = Vector(1, 2, 3)
}

D.a(0)  // works
D.b(0)  // does not work

// object can stand for no-paren method

trait E
trait F { def f:   E }
trait G { def f(): E }

object H extends F {
  object f extends E  // works
}

object I extends G {
  object f extends E  // does not work
}

Thus in terms of regularity of the language, it makes sense to have the distinction (especially for the last shown case).