Function parameter types and =>
What exactly that declaration of method parameter means:
def myFunc(param: => Int) = param
What is meaning of => in upper definition?
Answer
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 functions, 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 x function passed as a parameter).