Public functions should have an explicit return type

Prefer this:

def someF(par1: T1, par2: T2): Result = {
  ???
}

To this:

def someF(par1: T1, par2: T2) = {
  ???
}

Yeah, type inference on the result of a function is great and all, but for public methods:

it's not OK to rely on an IDE or to inspect the implementation in order to see the returned type
Scala currently infers the most specialized type possible, because in Scala the return type on functions is covariant, so you might actually get a really ugly type back

For example, what is the returned type of this function:

def sayRunnable(name: String) = new Runnable {
  def sayIt() = println(s"Hello, $name")
  def run() = sayIt()
}

Do you think it's Runnable? Wrong, it's Runnable{def sayIt(): Unit}.