Does Java support Currying?

Question

I was wondering if there is any way to pull that in Java. I think it is not possible without native support for closures.

Answer 1

Currying a method is always possible in Java, but it does not support it in a standard way. Trying to achieve this is complicated and makes the code pretty unreadable. Java is not the appropriate language for this.

Answer 2

There are a lot of options for Currying with Java 8. Function type Javaslang and jOOλ both offering Currying out of the box (I think this was an oversight in the JDK), and Cyclops Functions module has a set of static methods for Currying JDK Functions and method references. E.g.

  Curry.curry4(this::four).apply(3).apply(2).apply("three").apply("4");

  public String four(Integer a,Integer b,String name,String postfix){
    return name + (a*b) + postfix;
 }

'Currying' is also available for Consumers. E.g to return a method with 3 params, and 2 of those already applied we do something similar to this

 return CurryConsumer.curryC3(this::methodForSideEffects).apply(2).apply(2);

Javadoc

Answer 3

Yes, see the code example for yourself:

import java.util.function.Function;

public class Currying {

    private static Function<Integer, Function<Integer,Integer>> curriedAdd = a -> b -> a+b ;

    public static void main(String[] args) {

        //see partial application of parameters
        Function<Integer,Integer> curried = curriedAdd.apply(5);
        //This partial applied function can be later used as
        System.out.println("ans of curried add by partial application: "+ curried.apply(6));
        // ans is 11

        //JS example of curriedAdd(1)(3)
        System.out.println("ans of curried add: "+ curriedAdd.apply(1).apply(3));
        // ans is 4

    }

}

This is simple example with curriedAdd being a curried function which returns another function, and this can be used for partial application of parameters as stored in curried which is a function in itself. This is now later applied fully when we print it on screen.

Moreover, later you can see how you can use it in kind of JS style as

curriedAdd.apply(1).apply(2) //in Java
//is equivalent to 
curriedAdd(1)(2) // in JS

Answer 4

The advantage of using Currying in Java 8 is that it lets you define high order functions and then pass a first order function and function arguments in a chained, elegant way.

Here is an example for Calculus, the derivative function.

1. Lets define the derivative function approximation as (f(x+h)-f(x))/h. This will be the high order function
2. Let's calculate the derivative of 2 different functions, 1/x, and the standardized gaussian distribution

    package math;

    import static java.lang.Math.*;
    import java.util.Optional;
    import java.util.function.*;

    public class UnivarDerivative
    {
      interface Approximation extends Function<Function<Double,Double>, 
      Function<Double,UnaryOperator<Double>>> {}
      public static void main(String[] args)
      {
        Approximation derivative = f->h->x->(f.apply(x+h)-f.apply(x))/h;
        double h=0.00001f;
        Optional<Double> d1=Optional.of(derivative.apply(x->1/x).apply(h).apply(1.0)); 
        Optional<Double> d2=Optional.of(
        derivative.apply(x->(1/sqrt(2*PI))*exp(-0.5*pow(x,2))).apply(h).apply(-0.00001));
        d1.ifPresent(System.out::println); //prints -0.9999900000988401
        d2.ifPresent(System.out::println); //prints 1.994710003159016E-6
      }
    }

Answer 5

While you can do Currying in Java, it is ugly (because its not supported) In Java is it simpler and faster to use plain loops and simple expressions. If you post an example of where you would use currying, we can suggest alternatives which do the same thing.

Answer 6

One can emulate currying with Java 7 MethodHandles: http://www.tutorials.de/threads/java-7-currying-mit-methodhandles.392397/

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;

public class MethodHandleCurryingExample {
    public static void main(String[] args) throws Throwable {
        MethodHandles.Lookup lookup = MethodHandles.lookup();
        MethodHandle sum = lookup.findStatic(Integer.class, "sum", MethodType.methodType(int.class, new Class[]{int.class, int.class}));
        //Currying
        MethodHandle plus1 = MethodHandles.insertArguments(sum,0,1);
        int result = (int) plus1.invokeExact(2);
        System.out.println(result); // Output: 3
    }
}