java11-vavr-function-lifting

Examples of vavr function lifting.

Reference: https://www.vavr.io/vavr-docs/#_lifting

preface

A partial function from X to Y is a function f: X′ → Y, for some X′ c X. For x e X\X′ function is undefined.

In programming, if partial function is called with a disallowed input value, it will typically throw an exception.

In programming - we lift function f to f′: X -> Option<Y> in such a manner:

• f′(x).get() = f(x) on X′
• f′(x) = Option.none() for x e X\X′

vavr

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);
  

• lifting function with Try

  Function2<Integer, Integer, Integer> divide = (a, b) -> a / b;
  
  Function2<Integer, Integer, Try<Integer>> lifted = Function2.liftTry(divide);
  

project description

1. suppose we have:
   • BlockedUser with a method to activate it:

     @Value
     @Builder
     class BlockedUser {
         int id;
         int warn;
         LocalDate banDate;
         
         ActiveUser activate(Clock clock) {
             if (warn > 10) {
                 throw new BusinessException("id = " + id + ": warns has to be <= 10");
             }
             if (ChronoUnit.DAYS.between(banDate, LocalDate.now(clock)) < 14) {
                 throw new BusinessException("id = " + id + "minimum ban time is 14 days!");
             }
             return ActiveUser.builder().id(id).build();
         }
     }
     

   • simple ActiveUser

     @Value
     @Builder
     class ActiveUser {
         int id;
     }
     

2. suppose we cannot modify class BlockedUser
3. we have Stream of BlockedUsers and we want to activate them (if possible) and save to the database
   • activate throws exceptions, so we lift that function (FunctionLiftingTest - lift()):

     //        given
             ActiveUserRepository activeUserRepository = new ActiveUserRepository();
     
             var cannotBeActive = BlockedUser.builder()
                     .id(1)
                     .banDate(LocalDate.parse("2014-10-12"))
                     .warn(15)
                     .build();
     
             var canBeActive = BlockedUser.builder()
                     .id(2)
                     .banDate(LocalDate.parse("2016-10-12"))
                     .warn(0)
                     .build();
             
             var now = Clock.fixed(Instant.parse("2016-12-03T10:15:30Z"), ZoneId.systemDefault());
     
     //        when
             Stream.of(cannotBeActive, canBeActive)
                     .map(Function1.lift(x -> x.activate(now)))
                     .forEach(option -> option.peek(activeUserRepository::add));
     
     //        then
             assertTrue(activeUserRepository.existsAll(List.of(2)));
     

4. we have Stream of BlockedUsers and we want to activate them (if possible) and save to the database or generate report of exceptions occurred during activation (FunctionLiftingTest - liftTry())

   //        given
           ActiveUserRepository activeUserRepository = new ActiveUserRepository();
   
           var cannotBeActive = BlockedUser.builder()
                   .id(1)
                   .banDate(LocalDate.parse("2014-10-12"))
                   .warn(15)
                   .build();
   
           var canBeActive = BlockedUser.builder()
                   .id(2)
                   .banDate(LocalDate.parse("2016-10-12"))
                   .warn(0)
                   .build();
   
           var now = Clock.fixed(Instant.parse("2016-12-03T10:15:30Z"), ZoneId.systemDefault());
   
           List<String> fails = new LinkedList<>();
   
   //        when
           Stream.of(cannotBeActive, canBeActive)
                   .map(Function1.liftTry(x -> x.activate(now)))
                   .forEach(tryF -> tryF
                           .onSuccess(activeUserRepository::add)
                           .onFailure(exception -> fails.add(exception.getMessage())));
   
   //        then
           assertThat(activeUserRepository.count(), is(1));
           assertTrue(activeUserRepository.existsAll(List.of(2)));
   
   //        and
           assertThat(fails, hasSize(1));
           assertThat(fails.get(0), is("id = 1: warns has to be <= 10"));