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:
- different arguments of a function are calculated at different times. (Example 1)
- 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)
