Currying Functions in Scala

Currying is named after Haskell Curry, an American mathematician. He is known for his work in combinatory logic.

Currying is a means of transforming a function that takes more than one argument into a chain of calls to functions, each of which takes a single argument.

Let us consider the function below to calculate the final price of the product. The function takes in 3 parameters:

• VAT (vat) for the region
• Service charge (serviceCharge) of the shop
• Product price (productPrice)

def finalPrice(vat: Double, 
               serviceCharge: Double, 
               productPrice: Double): Double = 
productPrice + productPrice*serviceCharge/100 + productPrice*vat/100

But if you think about the function finalPrice again, a shopkeeper has to provide all the above values time and again whenever he wants to calculate the final price. Of course, that ignores the fact that the values of:

• VAT is already defined for a country
• Service charge for a shop is constant

So we will try to make life of our client a little bit easier. Let us define curried finalPrice:

def finalPriceCurried(vat: Double)
(serviceCharge: Double)
(productPrice: Double): Double = 
productPrice + productPrice*serviceCharge/100 + productPrice*vat/100

We are taking this approach because our vat and serviceCharge will not change very often. So, let's use currying to split our method. We will declare a new val: vatApplied. I will provide the value of vat to the finalPriceCurried method and assign it to vatApplied.

val vatApplied = finalPriceCurried(20) _

Next, we will provide a service charge to my vatApplied val, and we will leave the price to be provided by the shopkeeper whenever they need it.

val serviceChargeApplied = vatApplied(12.5)

Let us test our serviceChargeApplied function to calculate the final price of the product.

val finalProductPrice = serviceChargeApplied(120)

finalProductPrice: Double = 159.0

We have reduced our method from accepting 3 parameters to accept one parameter. So, we have split our method in such a way that we don't have to provide all the arguments at the same time. I can provide these arguments whenever they are available. This transformation is called currying.

We can also convert our existing methods to curry methods using function curried method. I have seen currying used a lot in my production code. It is always good to understand how these things work. You can refer to the video below to understand the concept in more detail and to check out few more examples.