Functional Error Handling With ArrowKT

Functional error handling with Kotlin
Abolishing try-catch blocks and making your code more readable using functional programming

1. Functional error handling with Arrow Uriel Salischiker - urielsalis.com
2. Requirements 1. Turn on TV 2. Watch the movie 3. Write the review of the movie
3. Model data class TV data class MovieOpinion data class Review fun turnOn(): TV = TODO() fun watch(): MovieOpinion = TODO() fun writeReview(): Review = TODO()
4. Exceptions fun turnOn(): TV = throw new RuntimeException("RemoteNotFound") fun watch(): MovieOpinion = throw new NotInNetflixException("NotInNetflix") fun writeReview(): Review = Review()
5. Issues with Exceptions 1. Breaks referential transparency 2. Broken GOTO 3. Try catch blocks 4. Costly to construct
6. Poor choice for - Modelling absence - Known alternate paths - Async - No access to source code
7. OK for - Fatal errors that you can’t recover from - Contributor to JVM internals - Want to create chaos and overthrow the government - This talk - You know what you are doing
8. So how do we model them?
9. ARROW
10. Option fun turnOn(): Option<TV> = none() fun watch(): Option<MovieOpinion> = some(MovieOpinion()) fun writeReview(): Review? = null
11. Either sealed class ReviewError object RemoteNotFound: ReviewError object NotInNetflix: ReviewError fun turnOn(): Either<RemoteNotFound, TV> = RemoteNotFound.left() fun watch(): Either<NotInNetflix, MovieOpinion> = MovieOpinion.right() fun writeReview(): Review = Review()
12. Either suspend fun writeReview() = either { val tv = turnOn().bind() val opinion = watch().bind() writeActualReview(opinion) } writeReview().fold( { println("Review: $it") }, { when(it) { RemoteNotFound -> println("Remote") NotInNetflix -> println("Netflix") }} )
13. Validated 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 $value") data class NotAnEmail(val reasons: Nel<ValidationError>) : ValidationError("Not a valid email") } data class FormField(val label: String, val value: String) data class Email(val value: String)
14. Validated sealed class Strategy { object FailFast : Strategy() object ErrorAccumulation : Strategy() } /** Abstracts away invoke strategy **/ object Rules { private fun FormField.contains(needle: String): ValidatedNel<ValidationError, FormField> = if (value.contains(needle, false)) validNel() else ValidationError.DoesNotContain(needle).invalidNel() private fun FormField.maxLength(maxLength: Int): ValidatedNel<ValidationError, FormField> = if (value.length <= maxLength) validNel() else ValidationError.MaxLength(maxLength).invalidNel() private fun FormField.validateErrorAccumulate(): ValidatedNel<ValidationError, Email> = contains("@").zip( Semigroup.nonEmptyList(), // accumulates errors in a non empty list, can be omited for NonEmptyList maxLength(250) ) { _, _ -> Email(value) }.handleErrorWith { ValidationError.NotAnEmail(it).invalidNel() } /** either blocks support binding over Validated values with no additional cost or need to convert first to Either **/ private fun FormField.validateFailFast(): Either<Nel<ValidationError>, Email> = either.eager { contains("@").bind() // fails fast on first error found maxLength(250).bind() Email(value) } operator fun invoke(strategy: Strategy, fields: List<FormField>): Either<Nel<ValidationError>, List<Email>> = when (strategy) { Strategy.FailFast -> fields.traverseEither { it.validateFailFast() } Strategy.ErrorAccumulation -> fields.traverseValidated(Semigroup.nonEmptyList()) { it.validateErrorAccumulate() }.toEither() } } https://arrow-kt.io/docs/patterns/error_handling/ Boilerplate
15. Validated val fields = listOf( FormField("Invalid Email Domain Label", "nowhere.com"), //this fails FormField("Too Long Email Label", "nowheretoolong${(0..251).map { "g" }}") FormField("Valid Email Label", "getlost@nowhere.com") ) Rules(Strategy.FailFast, fields) // Either.Left(NonEmptyList(DoesNotContain(value=@)))
16. Validated val fields = listOf( FormField("Invalid Email Domain Label", "nowhere.com"), //this fails FormField("Too Long Email Label", "nowheretoolong${(0..251).map { "g" }}") FormField("Valid Email Label", "getlost@nowhere.com") ) Rules(Strategy.ErrorAccumulation, fields) /* Either.Left(NonEmptyList( NotAnEmail(reasons=NonEmptyList(DoesNotContain(value=@))), NotAnEmail(reasons=NonEmptyList(DoesNotContain(value=@), MaxLength(value=250))) ))*/
17. TLDR - Modelling absence -> Option / Nullable - Known alternate paths -> Either - Validations -> Either/Validated
18. Questions?