method that return function in scala unclear syntax

Question

I'm new to Scala but I've been working with Java for a long time now so programming is not new to me. Anyway I read a some books on Scala and came across an example that a method returns a function. Now returning a function from method is not new since Java 8 support lambda expression but I can't understand the following example:

def lowIt(inputValue: String): Double => Double = {
  if (inputValue == "Lower") x => x * 0.85
  else x => x
}

I don't understand where x comes from, I would expect the following:

def lowIt(inputValue: String): Double => Double = x => {
   if (inputValue == "Lower") x * 0.85
  else x
}

I don't get the first way of writing the above method. Thank u in advance

Andrey Tyukin · Accepted Answer · 2018-04-21 12:39:04Z

If it seems more intuitive to you, you could start with the second version: it behaves the same as the first one. However, if you look at it closely, you will notice that the inputValue captured by the closure never changes, so that your function

x => { if (inputValue == "Lower") x * 0.85 else x }

always takes either the then-branch, or the else-branch. But then you don't really have to make the if-else comparison for every single x. Instead, you can make the decision just once.

In case that inputValue == "Lower" holds, your function becomes

x => { if (true) x * 0.85 else x } 
x => x * 0.85
_ * 0.85

In the case that inputValue != "Lower", your function is just

x => { if (false) x * 0.85 else x }
x => x
identity

Thus, starting with your own version, after a few simple rewriting steps, the function is simplified to

def lowIt(inputValue: String): Double => Double = {
  if (inputValue == "Lower") x => x * 0.85
  else x => x
}

or even shorter:

def lowIt(inputValue: String): Double => Double = {
  if (inputValue == "Lower") _ * 0.85
  else identity
}

The rewritten versions are more efficient, because the if does not have to be evaluated every time. The last version might even be a bit clearer, because the identity-function is written down as one non-simplifiable "thing" that can be returned.

lev · Accepted Answer · 2018-04-21 11:38:07Z

1

you can rewrite that function like this:

def lowIt(inputValue: String): Double => Double = {
  if (inputValue == "Lower") {(x: Double) => x * 0.85 }
  else {(x: Double) => x}
}

the 2 return values are functions, and x is the input of the function

answered Apr 21, 2018 at 11:38

lev

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1 Comment

Seth Tisue Over a year ago

you can replace {(x: Double) => x} with identity

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