Functional Programming in Java Parts 2 and 3

Curryable.java

public abstract class Curryable<T, F> {
    public abstract Function1<F, T> curry();

    public F apply(T t) {
        return curry().apply(t);
    }
}

Function2_and_3_Curryable.java

public abstract class Function2<R, T1, T2>
        extends Curryable<T1, Function1<R,T2>> {
    public abstract R apply(final T1 i1, final T2 i2);

    @Override
    public Function1<Function1<R, T2>, T1> curry() {
        return new Function1<Function1<R, T2>, T1>() {
            public Function1<R, T2> apply(final T1 i1) {
                return new Function1<R, T2>() {
                    @Override
                    public R apply(final T2 i2) {
                        return Function2.this.apply(i1, i2);
                    }
                };
            }
        };
    }
}

public abstract class Function3<R, T1, T2, T3>
        extends Curryable<T1, Function2<R, T2, T3>> {
    public abstract R apply(final T1 i1, final T2 i2, T3 i3);

    @Override
    public Function1<Function2<R, T2, T3>, T1> curry() {
        return new Function1<Function2<R, T2, T3>, T1>() {
            public Function2<R, T2, T3> evaluate(final T1 i1) {
                return new Function2<R, T2, T3>() {
                    @Override
                    public R apply(final T2 i2, final T3 i3) {
                        return Function3.this.apply(i1, i2, i3);
                    }
                };
            }
        };
    }
}

Function2and3.java

public abstract class Function2<R, T1, T2> extends Curryable<T1, Function1<R,T2>> {
    public abstract R evaluate(final T1 i1, final T2 i2);

    @Override
    public Function1<Function1<R, T2>, T1> curry() {
        return new Function1<Function1<R, T2>, T1>() {
            public Function1<R, T2> evaluate(final T1 i1) {
                return new Function1<R, T2>() {
                    @Override
                    public R evaluate(final T2 i2) {
                        return Function2.this.evaluate(i1, i2);
                    }
                };
            }
        };
    }

    public final Function0<R> apply(final T1 i1, final T2 i2) {
        return apply(i1).apply(i2);
    }
}

public abstract class Function3<R, T1, T2, T3> extends Curryable<T1, Function2<R, T2, T3>>{
    public abstract R evaluate(final T1 i1, final T2 i2, T3 i3);

    @Override
    public Function1<Function2<R, T2, T3>, T1> curry() {
        return new Function1<Function2<R, T2, T3>, T1>() {
            public Function2<R, T2, T3> evaluate(final T1 i1) {
                return new Function2<R, T2, T3>() {
                    @Override
                    public R evaluate(final T2 i2, final T3 i3) {
                        return Function3.this.evaluate(i1, i2, i3);
                    }
                };
            }
        };
    }

    public final Function0<R> apply(final T1 i1, final T2 i2, final T3 i3) {
        return apply(i1).apply(i2).apply(i3);
    }
}

MemoizationUtils_2_and_3.java

public class MemoizationUtils {

    /* ... previous definition of memoize for Function1... */

    public static <R, T1, T2> MemoizedFunction2<R, T1, T2>
      memoize(final Function2<R, T1, T2> f) {
        return new MemoizedFunction2<R, T1, T2>() {
            @Override
            public R evaluate(Function2<R, T1, T2> self,
                              final T1 i1, final T2 i2) {
                return f.evaluate(this, i1, i2);
            }
        };
    }
    
    public static <R,T1,T2,T3> MemoizedFunction3<R, T1, T2, T3> 
      memoize(final Function3<R, T1, T2, T3> f) {
        return new MemoizedFunction3<R, T1, T2, T3>() {
            @Override
            public R evaluate(Function3<R, T1, T2, T3> self,
                              final T1 i1, final T2 i2, final T3 i3) {
                return f.evaluate(this, i1, i2, i3);
            }
        };
    }
}

PartialApplicationExample.java

System.out.println("4 + 5 = " + add.apply(4).apply(5));

// More explicitly, we can capture the resulting function in a variable:
final Function1 addFour = add.apply(4);
System.out.println("4 + 5 = " + addFour.apply(5));