Scala Language Currying When to use Currying


Example

Currying is the technique of translating the evaluation of a function that takes multiple arguments into evaluating a sequence of functions, each with a single argument.

This is normally useful when for example:

  1. different arguments of a function are calculated at different times. (Example 1)
  2. different arguments of a function are calculated by different tiers of the application. (Example 2)

Example 1

Let's assume that the total yearly income is a function composed by the income and a bonus:

val totalYearlyIncome:(Int,Int) => Int =  (income, bonus) => income + bonus

The curried version of the above 2-arity function is:

val totalYearlyIncomeCurried: Int => Int => Int = totalYearlyIncome.curried

Note in the above definition that the type can be also viewed/written as:

Int => (Int => Int)

Let's assume that the yearly income portion is known in advance:

val partialTotalYearlyIncome: Int => Int = totalYearlyIncomeCurried(10000)

And at some point down the line the bonus is known:

partialTotalYearlyIncome(100)

Example 2

Let's assume that the car manufacturing involves the application of car wheels and car body:

val carManufacturing:(String,String) => String = (wheels, body) => wheels + body

These parts are applied by different factories:

class CarWheelsFactory {
  def applyCarWheels(carManufacturing:(String,String) => String): String => String =
          carManufacturing.curried("applied wheels..")
}
    
class CarBodyFactory {
  def applyCarBody(partialCarWithWheels: String => String): String = partialCarWithWheels("applied car body..")
}

Notice that the CarWheelsFactory above curries the car manufacturing function and only applies the wheels.

The car manufacturing process then will take the below form:

val carWheelsFactory = new CarWheelsFactory()
val carBodyFactory   = new CarBodyFactory()

val carManufacturing:(String,String) => String = (wheels, body) => wheels + body
  
val partialCarWheelsApplied: String => String  = carWheelsFactory.applyCarWheels(carManufacturing)
val carCompleted = carBodyFactory.applyCarBody(partialCarWheelsApplied)