Validation: Accumulating errors and failing fast in Arrow with `ApplicativeError`

validation.kt

import arrow.*
import arrow.core.*
import arrow.typeclasses.*
import arrow.data.*

sealed class ValidationError(val msg: String)
data class DoesNotContain(val value: String) : ValidationError("Did not contain $value")
data class MaxLength(val value: Int) : ValidationError("Exceeded length of $value")
data class NotAnEmail(val reasons: Nel<ValidationError>) : ValidationError("Not a valid email")

data class FormField(val label: String, val value: String)
data class Email(val value: String)

sealed class Rules<F>(A: ApplicativeError<F, Nel<ValidationError>>) : ApplicativeError<F, Nel<ValidationError>> by A {

  private fun FormField.contains(needle: String): Kind<F, FormField> =
    if (value.contains(needle, false)) just(this)
    else raiseError(DoesNotContain(needle).nel())

  private fun FormField.maxLength(maxLength: Int): Kind<F, FormField> =
    if (value.length <= maxLength) just(this)
    else raiseError(MaxLength(maxLength).nel())

  fun FormField.validateEmail(): Kind<F, Email> =
    map(contains("@"), maxLength(250), {
      Email(value)
    }).handleErrorWith { raiseError(NotAnEmail(it).nel()) }

  object ErrorAccumulationStrategy :
    Rules<ValidatedPartialOf<Nel<ValidationError>>>(Validated.applicativeError(NonEmptyList.semigroup()))
  
  object FailFastStrategy :
    Rules<EitherPartialOf<Nel<ValidationError>>>(Either.applicativeError())
  
  companion object {
    infix fun <A> failFast(f: FailFastStrategy.() -> A): A = f(FailFastStrategy)
    infix fun <A> accumulateErrors(f: ErrorAccumulationStrategy.() -> A): A = f(ErrorAccumulationStrategy)
  }

}



object RulesTest {
  @JvmStatic
  fun main(args: Array<String>): Unit {

    Rules accumulateErrors  {
      listOf(
        FormField("Invalid Email Domain Label", "nowhere.com"),
        FormField("Too Long Email Label", "nowheretoolong${(0..251).map { "g" }}"),
        FormField("Valid Email Label", "getlost@nowhere.com")
      ).forEach { println(it.validateEmail()) }
    }
//    Invalid(e=NonEmptyList(all=[NotAnEmail(reasons=NonEmptyList(all=[DoesNotContain(value=@)]))]))
//    Invalid(e=NonEmptyList(all=[NotAnEmail(reasons=NonEmptyList(all=[DoesNotContain(value=@), MaxLength(value=250)]))]))
//    Valid(a=Email(value=getlost@nowhere.com))

    Rules failFast  {
      listOf(
        FormField("Invalid Email Domain Label", "nowhere.com"),
        FormField("Too Long Email Label", "nowheretoolong${(0..251).map { "g" }}"),
        FormField("Valid Email Label", "getlost@nowhere.com")
      ).forEach { println(it.validateEmail()) }
    }
//    Left(a=NonEmptyList(all=[NotAnEmail(reasons=NonEmptyList(all=[DoesNotContain(value=@)]))]))
//    Left(a=NonEmptyList(all=[NotAnEmail(reasons=NonEmptyList(all=[DoesNotContain(value=@)]))]))
//    Right(b=Email(value=getlost@nowhere.com))

  }
}