208. Partially applying a Function

A Function that is partially applied is a Function that applies only a part of its arguments, returning another Function. For instance, here is a TriFunction (a functional function with three arguments) that contains the apply() method, next to two default methods that partially apply this function:

@FunctionalInterface
public interface TriFunction <T1, T2, T3, R> {
    
  R apply(T1 t1, T2 t2, T3 t3);
   
  default BiFunction<T2, T3, R> applyOnly(T1 t1) {
    return (t2, t3) -> apply(t1, t2, t3);
  }
    
  default Function<T3, R> applyOnly(T1 t1, T2 t2) {
    return (t3) -> apply(t1, t2, t3);
  }
}

As you can see, applyOnly(T1 t1) applies only the t1 argument and returns a BiFunction. On the other hand, applyOnly(T1 t1, T2 t2) applies only t1 and t2, returning a Function.

Let’s see how we can use these methods. For instance, let’s consider the formula (a+b+c)² = a²+b²+c²+2ab+2bc+2ca, which can...