One of the important features of a functional program is Function Currying.As we know java8 introduce lambda which brings some functional nature in java programming. We are blessed that by an intelligent use of Lambda we can create Function currying by own. Although Java8 does not have Currying facilities we can create it by ourselves. In this section, I will discuss the detail step How to create Currying in Java 8.
What is a Function Currying?
To understand the function currying let first understand what is a Partial function.
A partial function is the application of a part of a function.
To be very simple, Suppose I have a java method called add which takes two arguments as input then capability of applying one argument is called the Partial function.
Maybe you are astonished in this moment why the hell we need to do that?
Let me tell you, This is a very rich concept and it helps you to write a rich API and reduce the Repeating of yourself (DRY). It actually eliminates the conceptual duplication.
Now I am trying to describe it with a example so you can understand the above paragraph well.
Suppose, I want to write a generic compute function which takes two inputs and takes a BiFunction (Java 8 functional Interface) so I can pass behavior on the fly which will be applied to the two inputs argument.
Example
public Integer compute(Integer a, Integer b,BiFunction<Integer,Integer,Integer> function){
return function.apply(a,b);
}
By this compute method I can apply add, multiply, division whatever on these passing parameters a and b.
So I will call the Compute function in following way
BiFunction<Integer,Integer,Integer> addFunction = (a,b)> a+b;
Integer summation = fc.compute(50, 20, addFunction);
System.out.println(“Summation is ” + summation);
The output will be 70, as BiFunction called addFunction which do the adding.
Now suppose we have to build another logic, Say I pass a number then a base will automatically apply to it. The base value is 10.
So if the base is 10 and I pass 20 then the output will be 10 + 20 =30(base + actual parameter).
How would you write this function?
Very easy isnâ€™t it, we should write it in the following manner
public Integer baseCompute(Integer value,Function<Integer,Integer> function){
return function.apply(value);
}
It takes a value and a Function which takes one Integer as input and return another Integer So that this function takes the passing value apply base to it and return the resultant value.
We call this baseCompute method by the following call,
Function<Integer,Integer> baseFunction = (value)> 10 + value;//declare base
Integer baseResult = fc.baseCompute(20,baseFunction);
System.out.println(“Base computation ” + baseResult);
The output will be 30.
So far so good, But if you are a Clean coder you are not happy with it because you observe
BiFunction<Integer,Integer,Integer> addFunction = (a,b)> a+b;
Function<Integer,Integer> baseFunction = (value)> 10 + value;
The addFunction and baseFunction body are same, so you duplicate your code breaking the DRY principle. So you must search for an option where you can reuse the same addFunction instead of writing a new baseFunction, But the problem is the Structure of addFunction is different from the base function. addFunction takes two parameters and base takes one, But conceptually both are doing same work,
addFunction add two parameters, baseFunction takes one parameter and add base value internally.
Think How you can solve the problem while I taking a short break for breakfast.
We can solve this problem, by fixing a parameter in the bifunction. As we know the base value is 10 so if we apply that value and take another value from caller then we can reuse the BiFunction.
So we could rewrite the baseCompute method like following
public Integer applyPartialInternalCompute(Integer value,BiFunction<Integer,Integer,Integer> function){
return function.apply(10,value);
}
And calls the above method like following
BiFunction<Integer,Integer,Integer> addFunction = (a,b)> a+b;
Integer baseInternal = fc.applyPartialInternalCompute(20, addFunction);
System.out.println(“Base Internal ” + baseInternal);
Congrats you have written your first partial function, Where you fixed you first parameter and takes a parameter from Caller.
It is the essence of Partial function apply a partial part of the function and returns a new function which can take remaining arguments. So it is consists of two thing

Applying a part.

Return a new function after applying one argument.
Mathametically
f ( x , y ) = x+y
After applying the first parameter of function
f(2,y)=N(y)=2+y; where N(Y) is a new function returned after currying.
But what we did in our applyPartialInternalCompute method is not returning a new function or the intermediate function after partial applying the first value Which is base value, so it is not properly curryed this is very basic version, Let see how can we improve the function so we can get the new intermediate function after applying the first argument base.
public Function<Integer,Integer> applyPartialExternalCompute(Integer base,BiFunction<Integer,Integer,Integer> function){
return value>function.apply(base,value);
}
This is a more sophisticated version, It takes a Bifunction and a base value, Then apply the base value to the BiFunction and return a new Function which takes One parameter from the caller and returns another Integer.
So this function works as an adapter, It takes Bifunction applying a parameter or fixing a parameter and returns new Intermediate function to work upon,
BiFunction<Integer,Integer,Integer> = Fixed Base value + Function<Integer,Integer>
Please note that the returning function is looks like of our baseFunction Function<Integer,Integer> baseFunction = (value)> 10 + value;//declare base
This is called Function Currying, so we can use the BiFunction at fullest, So a BiFunction can take two parameters also it serves any functionality which takes one argument by fixing the first one.
How to call the applyPartialExternalCompute method,
Integer basepartial= fc.applyPartialExternalCompute(10, addFunction).apply(20);
System.out.println(“Base External ” + basepartial);
Our above solution is very good, and we properly use function Currying to remove conceptual duplication but wait there is still some glitch in the method we have written.
It takes Bifunction and a value and returns a Function, So every time we invoke this method it applies the parameter to BiFunction and return a new Function, But why should we perform two steps every time if we got the readymade intermediate function then we just apply our values,
Suppose there is a function which takes three arguments then we have to write two adapter functions one takes trifunction and return a Bifunction and another takes Bifunction and return a function,
So if parameter values are n we have to write n1 adaptor methods. Which is again a duplication, So if the Curry function itself generates all intermediate structures then it will be a full proof solution lets implement this,
public Function<Integer,Function<Integer,Integer>> applyCurryingCompute(BiFunction<Integer,Integer,Integer> function){
return value>(base>function.apply(base,value));
}
Yes, we are waiting for this implementation, and it is the Crux of this article, By above Curry function we should have every intermediate function and we can do any functionality by fixing others,
It takes a BiFunction which returns an Intermediate function which takes an argument and returns a function which again takes another argument and lastly returns an Integer.
Hard to understand,
Then see the following transformation to understand what actually going on
(x,y)>z = x>(y>z) BiFunction changes it structure after currying and create a new structure
(Integer,Integer)=Integer – > Integer =Integer>( Integer>Integer)
f(y)=x>f(x,y)=z where x>f(x,y) means x applied on Bifunction creates a function of y which is equivalent to Z
So (x>y)>z = x>(y>z).
How to call above applyCurryingCompute method
Integer baseCurry= fc.applyCurryingCompute(addFunction).apply(10).apply(20);
System.out.println(“Base Curry ” + baseCurry);
The full version of the program
package com.example.function.curry;
import java.util.function.BiFunction;
import java.util.function.Function;
public class FunctionCurrying {
public Integer compute(Integer a, Integer b,BiFunction<Integer,Integer,Integer> function){
return function.apply(a,b);
}
public Integer baseCompute(Integer value,Function<Integer,Integer> function){
return function.apply(value);
}
public Integer applyPartialInternalCompute(Integer value,BiFunction<Integer,Integer,Integer> function){
return function.apply(10,value);
}
public Function<Integer,Integer> applyPartialExternalCompute(Integer base,BiFunction<Integer,Integer,Integer> function){
return value>function.apply(base,value);
}
public Function<Integer,Function<Integer,Integer>> applyCurryingCompute(BiFunction<Integer,Integer,Integer> function){
return value>(base>function.apply(base,value));
}
public static void main(String[] args) {
FunctionCurrying fc = new FunctionCurrying();
BiFunction<Integer,Integer,Integer> addFunction = (a,b)> a+b;
Function<Integer,Integer> baseFunction = (value)> 10 + value;
Integer summation = fc.compute(50, 20, addFunction);
System.out.println(“Summation is ” + summation);
Integer baseResult = fc.baseCompute(20,baseFunction);
System.out.println(“Base computation ” + baseResult);
Integer baseInternal = fc.applyPartialInternalCompute(20, addFunction);
System.out.println(“Base Internal ” + baseInternal);
Integer basepartial= fc.applyPartialExternalCompute(10, addFunction).apply(20);
System.out.println(“Base External ” + basepartial);
Integer baseCurry= fc.applyCurryingCompute(addFunction).apply(10).apply(20);
System.out.println(“Base Curry ” + baseCurry);
}
}
Conclusion: Function Currying is a rich technique, In the functional program we use it often to get rid of conceptual duplication. You can use it in Java 8 now so Please use the concept wisely.
Function Curry in Java8