Contributing Scala “When to use Currying” in StackOverflow Documentation

A week or so ago I’ve contributed in StackOverflow Documentation an entry about Currying under the Scala Language tag. I’ve repeated the same exercise for Java Currying Documentation. This is how it goes:

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)
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