Difference Between Functions and Methods in Scala

Posted on November 5, 2016 by Marcin Kossakowski
Tags: 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:

Simple function:

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.

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.

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.

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

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.

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.

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