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

This is a library for currying and partial application in Java :

https://github.com/Ahmed-Adel-Ismail/J-Curry

It also supports destructuring Tuples and Map.Entry into method parameters, like for example passing a Map.Entry to a method that takes 2 parameters, so the Entry.getKey() will go to the first parameter, and the Entry.getValue() will go for the second parameter

More details in the README file

Answer 2

Currying and partial application is absolutely possible in Java, but the amount of code required will probably turn you off.

---

Some code to demonstrate currying and partial application in Java:

interface Function1<A, B> {
  public B apply(final A a);
}

interface Function2<A, B, C> {
  public C apply(final A a, final B b);
}

class Main {
  public static Function2<Integer, Integer, Integer> simpleAdd = 
    new Function2<Integer, Integer, Integer>() {
      public Integer apply(final Integer a, final Integer b) {
        return a + b;
      }
    };  

  public static Function1<Integer, Function1<Integer, Integer>> curriedAdd = 
    new Function1<Integer, Function1<Integer, Integer>>() {
      public Function1<Integer, Integer> apply(final Integer a) {
        return new Function1<Integer, Integer>() {
          public Integer apply(final Integer b) {
            return a + b;
          }
        };
      }
    };

  public static void main(String[] args) {
    // Demonstrating simple `add`
    System.out.println(simpleAdd.apply(4, 5));

    // Demonstrating curried `add`
    System.out.println(curriedAdd.apply(4).apply(5));

    // Curried version lets you perform partial application 
    // as demonstrated below.
    Function1<Integer, Integer> adder5 = curriedAdd.apply(5);
    System.out.println(adder5.apply(4));
    System.out.println(adder5.apply(6));
  }
}

---

FWIW here is the Haskell equivalent of above Java code:

simpleAdd :: (Int, Int) -> Int
simpleAdd (a, b) = a + b

curriedAdd :: Int -> Int -> Int
curriedAdd a b = a + b

main = do
  -- Demonstrating simpleAdd
  print $ simpleAdd (5, 4)

  -- Demonstrating curriedAdd
  print $ curriedAdd 5 4

  -- Demostrating partial application
  let adder5 = curriedAdd 5 in do
    print $ adder5 6
    print $ adder5 9

Answer 3

Currying requires to return a function. This is not possible with java (no function pointers) but we can define and return a type that contains a function method:

public interface Function<X,Z> {  // intention: f(X) -> Z
   public Z f(X x);
}

Now let's curry a simple division. We need a Divider:

// f(X) -> Z
public class Divider implements Function<Double, Double> {
  private double divisor;
  public Divider(double divisor) {this.divisor = divisor;}

  @Override
  public Double f(Double x) {
    return x/divisor;
  }
}

and a DivideFunction:

// f(x) -> g
public class DivideFunction implements Function<Double, Function<Double, Double>> {
  @Override
  public function<Double, Double> f(Double x) {
    return new Divider(x);
  }

Now we can do a curried division:

DivideFunction divide = new DivideFunction();
double result = divide.f(2.).f(1.);  // calculates f(1,2) = 0.5