On def and compiler magic.

Avada Kedavra! Vingardium Leviosa! myFunction _!

A great thing aboout map, flatMap and others is that they are higher-order-functions (i.e. functions that take other functions as parameters or return functions) allowing us to modularize our code and avoid the … ehm … function-as-parameter-hell™ (named after callback-hell).

List(1, 2, 3, 4).map{ number =>
	// ...
	// complex,
	// multiline and
	// useful elsewhere code
	// that manipulates each number

// can be rewritten extrapolating the function passed to map
def usefulFunction(number: Int) = ... // our complex function
List(1, 2, 3).map(usefulFunction)

But there is a catch, what if we wanted to use the same approach with a list of tuples? What could possibly go wrong?

def add(x: Int, y: Int) = x + y
val pairRange = (6 to 10) zip (4 to 8) // create a two numbers sequence and zip them

// error: type mismatch;
//  found   : (Int, Int) => Int
//  required: ((Int, Int)) => ?
//        pairRange.map(add)

What happened? There’s a small but substantial difference in place. Add takes two arguments while each item of pairRange is a pair. It’s clear now … right?

Not that mutch, but let’s take a look to the error. (Int, Int) => Int found, but ((Int, Int)) => ? required. The extra parenthesis tells us exactly where the problem is. It expects a function which takes only one parameter that is a pair, while our add function takes two parameters.

Quite a deep explaination. Jump to next section if you get bored.

One thing to keep in mind with Scala is that it is built on top of Java and the JVM that are not that functional-programming friendly. In contrast, let’s see two function declarations in OCaml (the same should apply to F#).

let add a b = a + b   // int -> int -> int
let sub(a,b) = a - b  // (int, int) -> int

In OCaml, by default, you define functions in the form of our add, that is a function f that takes a parameter (a) and returns another function f’ that takes a parameter (b) and returns a value. This is a powerful mechanism that allows you to play with cool things like partial application (note to self, write a post about it). Alternatively you can define functions, like sub, which take N parametrs as a tuple and return a value.

In Scala you usually define functions of the second type. Moreover, in Scala unlike OCaml, F#, Haskell, etc.. functions, under the JVM hood, are not first-class citizens. Hence each function is compiled to an object with an apply method which takes exactly the required parameters. So the error we’ve seen in the previous section.

How to handle the case

The Function type in Scala has a method called tupled that allows you to transform a function that takes N parameters in a function that takes as single argument a tuple with N elements. E.g. it means it converts a function in the form (Int, Int) => Int to one in the form ((Int, Int)) => Int. So it is likely to think that the solution to be:

// error: missing arguments for method add;

Damn it Scala!! What do you want more from me? Thing is, Scala runs on the JVM. In Java everything is an object. Hence each function we define with def must be a method of some object, but a method is not a function. We need a way to extract from a method its corresponding function. That is:

val addFunction = (add _) // it will have type (Int, Int) => Int = <function2>
// or shorter
pairRange.map((add _).tupled)

Be aware that this is not a part of the api but a language feature or … compiler magic.