kresult-core/io.kresult.core/MultiErrorKResult

MultiErrorKResult

typealias MultiErrorKResult<E, T> = KResult<List<E>, T>(source)

Typealias to represent KResult types that have a List type on the Failure side

Validation

MultiErrorKResult instances have validate and errors extension functions. They can be used for very easy, type-safe and side-effect-free object validation:

import io.kresult.core.KResult
import io.kresult.core.MultiErrorKResult
import io.kresult.core.errors
import io.kresult.core.validate
import io.kotest.matchers.shouldBe

data class User(val name: String, val age: Int, val active: Boolean)

enum class ValidationError(val message: String) {
  InvalidName("Name must not be empty"),
  IllegalAge("User must be over 18 years old"),
  Inactive("User must active"),
}

fun test() {
  val user: MultiErrorKResult<ValidationError, User> =
    KResult.Success(User(name = "Max", age = 17, active = false))

  val result: MultiErrorKResult<ValidationError, User> = user
    .validate(ValidationError.InvalidName) { u ->
      u.name.isNotBlank()
    }
    .validate(ValidationError.IllegalAge) { u ->
      u.age >= 18
    }
    .validate(ValidationError.Inactive) { u ->
      u.active
    }

  result.isFailure() shouldBe true
  result.errors().size shouldBe 2
  result.errors()[0] shouldBe ValidationError.IllegalAge
  result.errors()[1] shouldBe ValidationError.Inactive
}

Be careful with flatMap-ing validation results

If you need to flatMap a validation result (which is either a Success or a FailureWithValue), make sure that there are no successive validate calls. In the failure case of a flatMap operation, we lose the type information of FailureWithValue.value and therefore need to return a plain Failure object. This, will cause all successive validate calls to be ignored without being evaluated, as we have nothing to apply the predicate function to. While such scenarios compile, having validations that are never applied should be, at least semantically, considered bugs.

import io.kresult.core.KResult
import io.kresult.core.MultiErrorKResult
import io.kresult.core.flatMap
import io.kresult.core.validate
import io.kotest.matchers.shouldBe

fun test() {
  val num: MultiErrorKResult<String, Int> = KResult.Success(7)

  val result = num
    .validate("Must be greater zero") {
      it 0
    }
    .flatMap {
      // while this compiles...
      if (it % 3 == 0) {
        KResult.Success(it)
      } else {
        KResult.Failure(listOf("Must be modulus of 3"))
      }
    }
    .validate("Must be even") {
      // this will never be called, if the flatMap before evaluates to Failure
      it % 2 == 0
    }

  // we would expect a second error: "Must be even" here, but the last validation was never called
  result shouldBe KResult.Failure(listOf("Must be modulus of 3"))
}

Since

0.2.0

MultiErrorKResult

Validation

Be careful with flatMap-ing validation results

Since

See also