java11-vavr093-validation-workshop

project description

• https://softwaremill.com/javaslang-data-validation/
• https://www.youtube.com/watch?v=wJn4LHD6Y7o
• https://www.vavr.io/vavr-docs/#_validation
• https://github.com/mtumilowicz/java11-vavr-validation
• https://github.com/mtumilowicz/java11-jsr303-custom-validation
• in the workshop we will try to rewrite methods in the classes with naming convention: com.example.vavr.validation.workshop.*.*Workshop using hints and refactoring plan depicted below
• answers: com.example.vavr.validation.workshop.*.*Answer

refactoring plan:

1. change in build.gradle

   test {
       exclude '**/**AfterRefactor*'
   }
   

   to

   test {
       exclude '**/**BeforeRefactor*'
   }
   

   and we want to fix all failing tests
2. rewrite patterns using Validation instead of throwing exception
   • method: validateWorkshop in classes
     • Age
     • City
     • Email
     • Name
     • PostalCode
3. rewrite *Workshop classes using Validation instead of throwing exception and try-catch blocks (more hints in the classes)
   • NewAddressRequestValidatorWorkshop
   • NewPersonRequestValidatorWorkshop
   • PersonControllerWorkshop
4. delete
   • ErrorMessagesControllerAdvice
   • ValidationException
5. all tests **/*WorkshopAfterRefactor* should pass in this step

theory in a nutshell

• Validation is an applicative functor and facilitates accumulating errors
• when trying to compose Monads, the combination process will short circuit at the first encountered error as getting subsequent wrapped values depends on results of previous calculations
• Validation will continue processing and accumulate all errors (if any)
• it is especially useful when doing validation of multiple fields, say a web form, and you want to know all errors encountered, instead of one at a time
• contrary to Bean Validation standard (JSR-303 and JSR-349):
  • https://github.com/mtumilowicz/java11-jsr303-custom-validation
  • https://github.com/mtumilowicz/java-bean-validation2
  • we do not need any infrastructure providers (AOP, dynamic proxies, DI)
  • bad request is not rejected and can be easily handled in a reasonable way (for example send to patch service)
• interface Validation<E, T> extends Value<T>, Serializable
  • interface Value<T> extends Iterable<T>
• two implementations:
  • final class Valid<E, T> implements Validation<E, T> - contains valid data
  • final class Invalid<E, T> implements Validation<E, T> - is an error representation

conclusions in a nutshell

• creating
  • static <E, T> Validation<E, T> valid(T value)
  • static <E, T> Validation<E, T> invalid(E error)
  • example

    private static final IntPredicate PREDICATE = i -> i > 0;
    
    public static Validation<String, Age> validateAnswer(int age) {
        return PREDICATE.test(age)
                ? Validation.valid(new Age(age))
                : Validation.invalid("Age: " + age + " is not > 0");
    }
    

• combining
  • static <E, T1, ...> Builder<E, T1, ...> combine(Validation<E, T1> validation1, Validation<E, T2> validation2, ...)
  • public <R> Validation<Seq<E>, R> ap(FunctionN<T1, T2, ..., R> f)
  • example

    static Validation<Seq<String>, NewAddressCommand> validate(NewAddressRequest request) {
    
        return Validation
                .combine(
                        City.validateAnswer(request.getCity()),
                        PostalCode.validateAnswer(request.getPostalCode()))
                .ap((city, postalCode) -> NewAddressCommand.builder()
                        .city(city)
                        .postalCode(postalCode)
                        .build());
    }
    

  • up to 8 arguments
  • combine and functionN in ap should have the same number of params
  • valid type of combine params should be the same as type of arguments of functionN in ap
  • combine params order corresponds to the order of params in the functionN in ap
  • if all of Validations in combine are Valid, the ap(f) method maps all results to a single value using a function f
  • if at least one of Validations is Invalid, then ap(f) returns Invalid with all errors aggregated
  • for example, for 3 arguments ap looks like
    • ap in Builder class (Builder is returned by combine method)

      public <R> Validation<Seq<E>, R> ap(Function3<T1, T2, T3, R> f) {
          return v3.ap(v2.ap(v1.ap(Validation.valid(f.curried()))));
      }
      

    • ap in Validation class

        default <U> Validation<Seq<E>, U> ap(Validation<Seq<E>, ? extends Function<? super T, ? extends U>> validation) {
            Objects.requireNonNull(validation, "validation is null");
            if (isValid()) {
                if (validation.isValid()) {
                    final Function<? super T, ? extends U> f = validation.get();
                    final U u = f.apply(this.get());
                    return valid(u);
                } else {
                    final Seq<E> errors = validation.getError();
                    return invalid(errors);
                }
            } else {
                if (validation.isValid()) {
                    final E error = this.getError();
                    return invalid(List.of(error));
                } else {
                    final Seq<E> errors = validation.getError();
                    final E error = this.getError();
                    return invalid(errors.append(error));
                }
            }
        }
    

• consuming
  • pattern matching

    return Match(validation).of(
            Case($Valid($()), ...),
            Case($Invalid($()), ...)
    );
    

  • folding to single value

    return validation.fold(..., ....)
    

    • U fold(Function<? super E, ? extends U> fInvalid, Function<? super T, ? extends U> fValid)
• transforming
  • mapping

    validation.map(...)
        .mapError(...)
    

    • Validation<E, U> map(Function<? super T, ? extends U> f)
    • Validation<U, T> mapError(Function<? super E, ? extends U> f)
  • bimapping

    validation.bimap(..., ...)
    

    • Validation<E2, T2> bimap(Function<? super E, ? extends E2> errorMapper, Function<? super T, ? extends T2> valueMapper)
  • bimap vs fold - bimap returns Validation and fold returns any arbitrary type