How to read a chain of Scala function parameters

Question

I am trying to read the parameter-list of the following 2 functions:

1. def foo(action: => String => String) = "bar"
2. def foo(action: => () => String => String) = "bar"

1. A function named "foo" which receives a function named "action" which receives/returns ???
2. A function named "foo" which receives a function named "action" which returns a function which returns ???

Answer 1

1. action is a passed-by-name function that takes a String and returns a String.
2. action is a passed-by-name function that takes nothing to return a function that takes a String and returns a String

Now you might ask, "Well, what does it mean for a parameter to be passed-by-name?" Alright... that's a whole different can of worms. Basically, a passed by name parameter is only evaluated when it's used in the function, and every time that it's used in the function. What this allows for is something like short-circuiting, as follows

def orOperator(left: Boolean, right: => Boolean) : Boolean = if (left) true else right

In this case, the operator will short-circuit (and terminate without computing/evaluating right) if it finds left to be true.

So... what you have with these parameters is something similar. They are functions that do not evaluate—for some reason—unless/until they are named in the function body. I don't understand the motivation for that, but... that's how it is. I hope that helps.