java11-vavr093-partial-function-lifting-workshop

project description

• https://www.vavr.io/vavr-docs/#_lifting
• https://static.javadoc.io/io.vavr/vavr/0.9.3/io/vavr/PartialFunction.html
• https://github.com/mtumilowicz/java11-vavr-function-lifting
• in the workshop we will try to fix failing tests test/groovy/Workshop
• answers: test/groovy/Answers (same tests as in Workshop but correctly solved)

theory in a nutshell

• a partial function from X to Y is a function f: K → Y, for some K c X. For x e X\K function is undefined
• it generalizes the concept of a function f: X → Y by not forcing f to map every element of X to an element of Y
  • that means a partial function works properly only for some input values
  • in programming, if the function is called with a disallowed input value, it will typically throw an exception
    • in particular - every function that throws an exception is a partial function
• partial function (to set intuition)

  int do(int positive) {
      if (positive < 0) {
              throw new IllegalArgumentException("Only positive integers are allowed"); 
      }
      // other stuff
  }
  

• vavr's partial function interface

  public interface PartialFunction<T, R> {
      R apply(T t);
  
      boolean isDefinedAt(T value); // tests if a value is contained in the function's domain
  }
  

  • the caller is responsible for calling the method isDefinedAt() before this function is applied to the value
  • if the function is not defined for a specific value, apply() may produce an arbitrary result
    • in particular - random values
    • more specifically - it is not guaranteed that the function will throw an exception
  • do(int positive) mentioned above - rewritten with vavr:

    class RandomIdentity implements PartialFunction<Integer, Integer> {
        
        @Override
        public Integer apply(Integer o) {
            if (!isDefinedAt(o)) {
                throw new IllegalArgumentException("Only positive integers are allowed");
            }
            // other stuff
        }
    
        @Override
        public boolean isDefinedAt(Integer value) {
            return nonNull(value) && value > 0;
        }
    }
    

• In programming - we usually lift partial function f: (K c X) -> Y to g: X -> Option<Y> in such a manner:
  • a lifted function returns Some, if the function is invoked with allowed input values
    • g(x).get() = f(x) on K
  • a lifted function returns None instead of throwing an exception, if the function is invoked with disallowed input values
    • g(x) = Option.none() for x e X\K

conclusions in a nutshell

In vavr we have two approaches to lifting:

• lifting function with Option

  Function2<Integer, Integer, Integer> divide = (a, b) -> a / b;
  
  Function2<Integer, Integer, Option<Integer>> lifted = Function2.lift(divide);
  
  lifted.apply(1, 0) == Option.none()
  lifted.apply(4, 2) == Option.some(2)
  

• lifting function with Try

  Function2<Integer, Integer, Integer> divide = (a, b) -> a / b;
  
  Function2<Integer, Integer, Try<Integer>> lifted = Function2.liftTry(divide);
  
  lifted.apply(1, 0).failure
  lifted.apply(1, 0).cause.class == ArithmeticException
  lifted.apply(4, 2) == Try.success(2)
  

• Try is nearly always better, because it contains specific exception, which is often a very valuable information, we do not want to lose