Support Validation using Applicative Functor

Hey there, that’s a great library! But I see an issue, similar to the same issue that affects validation via annotations. It is not that simple to define a reusable Value Object class, that is itself composed by other Value Object classes.

Example in Kotlin:

data class Email private constructor(val value: String){
   companion object {
       fun of(value: String): Either<ConstraintViolations, Email> = ValidatorBuilder.of<Email>()
            .konstraint(Email::value) {
                email()
            }.build()
            .validateToEither(Email(emailValue))
   }
}
data class Phone private constructor(val value: String){
   ... similar factory method here
}
data class ContactsInfo(val email: Email, val phone: PhoneNumber){
   companion object {
       fun of(emailValue: String, phoneValue: String): Either<ConstraintViolations, ContactsInfo>{
          val email: Either<ConstraintViolations, Email> = Email.of(emailValue)
          val phone: Either<ConstraintViolations, PhoneNumber> = PhoneNumber.of(phoneValue)
          
          ... How to compose email and phone here to build a ContactsInfo()? ...
       }
   }
}

Build a ConcactsInfo like in the example is not possible as Either lacks of a flatMap method. But also if flatMap was there, it would short-circuit on email, and then we would miss an eventual error on phone, in the list of ConstraintViolations.

I see only two ways out, in the current version of Yavi:

1. making public the constructors of Email and PhoneNumber and having the of factory method return the ConstraintValidations, by calling validate instead of validateToEither. Then, every time check the value of the isValid property of ConstraintValidations. This solution is not explicit, neither type-safe, and drives to duplication.
2. Use exceptions.

My solution proposal is: let’s discuss a way of integrating Yavi with the Validation Monad of Konad in order to have an extension method like

fun <T> Validator<T>.validateToKonad(target: T): Validation<ConstraintViolations, T> =
    validate(target).run {
        if(isValid) target.success()
        else fail()
    }

With the Validation from Konad the example above would become

data class ContactsInfo(val email: Email, val phone: PhoneNumber){
   companion object {
       fun of(emailValue: String, phoneValue: String): Validation<ConstraintViolations, ContactsInfo> =
           ::ContactsInfo + Email.of(emailValue) + PhoneNumber.of(phoneValue)
   }
}

[making]

BTW, I think YAVI’s Arguments Validator can achieve the same outcome though it’s more verbose:

Validator<Email> emailValidator = ...;
Validator<PhoneNumber> phoneNumberValidator = ...;

Arguments2Validator<Email, PhoneNumber, ContactInfo> contactInfoValidator = ArgumentsValidatorBuilder.of(ContactInfo::new)
    .builder(b -> b.nest(Arguments1::arg1, "email", emailValidator)
                   .nest(Arguments2::args, "phoneNumber", phoneNumberValidator))
    .build();

Either<ConstraintViolations, ContactInfo> either = contactInfoValidator.validateArgs(email, phoneNumber);

[making]

YAVI 0.6.0 has been released though it’s not yet synced with Maven Central https://github.com/making/yavi/releases/tag/0.6.0

Final form looks like

Validator<Email> emailValidator = ...;
Validator<PhoneNumber> phoneNumberValidator = ...;

Validated<Email> emailValidated = emailValidator.applicative().validate(email);
Validated<PhoneNumber> phoneNumberValidated = phoneNumberValidator.applicative().validate(phoneNumber);

Validated<ContactInfo> contactInfoValidated = emailValidated.combine(phoneNumberValidated)
            .apply((em, ph) -> new ContactInfo(em, ph));
// or
Validated<ContactInfo> contactInfoValidated = Validations.combine(emailValidated, phoneNumberValidation)
		.apply((em, ph) -> new ContactInfo(em, ph));

boolean isValid = contactInfoValidated.isValid();

ContactInfo contactInfo = contactInfoValidated
        		.orElseThrow(violations -> new ConstraintViolationsException(violation));

HttpStatus status = contactInfoValidated
        		.fold(violations -> HttpStatus.BAD_REQUEST, contactInfo -> HttpStatus.OK);