Functions vs methods in Scala

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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
}

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