Functions vs methods in Scala

Solution 1:

Under the hood, there are other differences between functions and methods. Generally, a plain method generated less overhead than a function (which technically is an object with an apply method).

However, if you try not to care about those differences and think of def, val and var as fields with different semantics, then it’s simply that def evaluates every time it gets called while val evaluates only once.

So, a val isEven = isDivisibleBy(2) should call isDivisibleBy(2) during its definition and assign the result of isDivisibleBy(2). E.g. it replaces the k in

def isDivisibleBy(k: Int): Int => Boolean = i => i % k == 0

with 2 and assigns the result of the final expression (in this case there is only one expression):

val isEven: Int => Boolean = i => i % 2 == 0

def isEven on the other hand does no such evaluation and results in a call to isDivisibleBy(2) every time.

That means, later, when you execute the code, isEven(11) generates in case of a val

11 % 2 == 0

and in case of a def, you’ll have

isDivisibleBy(2)(11)

and only after evaluating isDivisibleBy you’ll get the result.

You can add some debug code to isDivisibleBy to see the difference:

def isDivisibleBy(k: Int): Int => Boolean = {
  println("evaluating isDivisibleBy")
  i => i % k == 0
}

Solution 2:

I'd like to address another point here. This defines isEven as a method:

def isEven = isDivisibleBy(2)

And this defines isEven as a method as well:

val isEven = isDivisibleBy(2)

In both cases, isEven is a method which, when called, return a function.

In the first case, isDivisible(2) is called every time isEven is called. For example, this calls isDivisible(2) three times:

def isEven = isDivisibleBy(2)
List(1,2,3).filter(isEven)

In the second case, isDivisible(2) is called once (at construction time, or when that line in a definition is executed), and that value is retrieved every time isEven is called. The following example calls isDivisible(2) one time only:

val isEven = isDivisibleBy(2)
List(1,2,3).filter(isEven)

Solution 3:

I think that the main pro of defining the function isEven as val is to show to audience that the function can be defined this way. Then it's clear, that a function is just an object like everything else in scala. But in the world of non-demonstrating programming, there's no need to write functions as vals.