Marcin Kossakowski

Marcin Kossakowski

Tech and stuff

05 Nov 2016

Difference Between Functions and Methods in Scala

Scala has a concept of functions and method definitions. These two concepts are often confused and it’s not always clear when to use which.

Consider this example.

Simple method:

def m1(x: Int) = x + x
m1(2)  // 4

Simple function:

val f1 = (x: Int) => x + x
f1(2)  // 4

Both m1 and f1 are called the same way and will produce the same result but when you look closer you will see that these are two different things.

f1  // Int => Int = <function1>
m1  // error: missing argument list for method m1...

Calling f1 without argument will give us the signature of anonymous function. Our anonymous functions is actually an instance of Function1[Int, Int] that means a function with 1 parameter of type Int and return value of type Int.

f1.isInstanceOf[Function1[Int, Int]]  // Boolean = true  

When calling m1 we will get an error. This is because m1 is not a value, it is a method, something that will produce a value when you call it by providing all required arguments.

Converting method into a function

Method can be converted into a proper function (often referred to as lifting) by calling method with underscore “_” after method name.

val f2 = m1 _  // Int => Int = <function1>

Alternatively you can supply a type and compiler will know what to do.

val f2: Int => Int = m1  // Int => Int = <function1>

Above method to function conversions will always create a new instance of Function1[Int, Int] which is an important observation.

Let’s consider following example. We are creating a sequence of tuples with type (Int, Int => Int). For each tuple we decided to convert our previously defined method into a function. The following will create 10 instances of a function that does the same thing.

for (i <- 0 until 10) yield i -> m1 _

Better approach would be to pass the same instance of the function for each tuple and avoid allocating memory for each instance as in previous approach.

for (i <- 0 until 10) yield i -> f1

When to use methods and when functions

  • use functions if you need to pass them around as parameters
  • use functions if you want to act on their instances, e.g. f1.compose(f2)(2)
  • use methods if you want to use default values for parameters e.g. def m1(age: Int = 2) = ...
  • use methods if you just need to compute and return