Question

What does it mean when someone says “functions in scala associate to the right“?

And, how will associativity work in case Int => Int => Int

Answer 1

The function type Int => Int => Int is equivalent to Int => (Int => Int). In other words, the => groups things on the right first, or, is right-associative.

The type Int => (Int => Int) defines a single-argument function that accepts an Int and whose return-type is function from Int to Int.

So what’s an example of such a function? We could write a function called sum with this type:

val sum: Int => Int => Int = {
  (a: Int) =>
    {
      (b: Int) =>
        a + b
    }
}

So sum takes an argument a and returns a new function that accepts and argument b and returns the sum of a and b:

scala> val sumWith3 = sum(3)         // `sum` takes an Int argument
sumWith3: Int => Int = <function1>   // notice the return type of `Int => Int`

scala> sumWith3(5)                   // takes an Int argument
res0: Int = 8                        // returned an Int