import java.util.function.*;

interface Currying {
  public static <T, U, R> Function<U, R> leftSection(BiFunction<T, U, R> f, T left) {
    return right -> f.apply(left, right);
  }

  public static <T, U, R> Function<T, R> rightSection(BiFunction<T, U, R> f, U right) {
    return left -> f.apply( left, right );
  }

	// public static ... flip(...) { ... }

	// public static ... toUnary(...) { ... }

  public static void main(String[] args) {
    BinaryOperator<Number> minus = ( x, y ) -> x.doubleValue() - y.doubleValue();
    System.out.println( leftSection( minus, 4 ).apply( 3 ) );
    System.out.println( rightSection( minus, 4 ).apply( 3 ) );

    // ... concat = ...
    System.out.println(
      leftSection( flip( concat ), "5" ).apply("10").equals(
        rightSection( concat, "5" ).apply( "10" ) ) );

    System.out.println(
      rightSection( minus, 4 ).apply( 3 ).equals(
        toUnary( flip( minus ) ).apply( 4 ).apply( 3 ) ) );
  }
}