Function parameter types and =>

This is so-called pass-by-name. It means you are passing a function that should return Int but is mostly used to implement lazy evaluation of parameters. It is somewhat similar to:

def myFunc(param: () => Int) = param

Here is an example. Consider an answer function returning some Int value:

def answer = {println("answer"); 40}

And two function, one taking Int and one taking Int by-name:

def eagerEval(x: Int) = {println("eager"); x;}
def lazyEval(x: => Int) = {println("lazy"); x;}

Now execute both of them using answer:

eagerEval(answer + 2)
> answer
> eager

lazyEval(answer + 2)
> lazy
> answer

The first case is obvious: before calling eagerEval() answer is evaluated and prints "answer"string. The second case is much more interesting. We are actually passing a function to lazyEval(). The lazyEval first prints "lazy" and evaluates the x parameter (actually, calls xfunction passed as a parameter).