Functional Error Handling with Cats

FUNCTIONALFUNCTIONAL
ERROR HANDLINGERROR HANDLING
WITH CATSWITH CATS
MARK CANLASMARK CANLAS ·· NE SCALA 2020NE SCALA 2020

OPTIONOPTION[_][_]
TRYTRY[_][_]
EITHEREITHER[_, _][_, _]

FUNCTORFUNCTOR[_][_]
APPLICATIVEAPPLICATIVE[_][_]
MONADMONAD[_][_]

PARSING STRINGSPARSING STRINGS
case class AncientRecord(
age: String,
name: String,
rateOfIncome: String
)
case class Citizen(
age: Int,
name: String,
rateOfIncome: Double
)
def parseRecord(rec: AncientRecord): Citizen

PARSING FUNCTIONSPARSING FUNCTIONS
def parseAge(s: String)
def parseName(s: String)
def parseRate(s: String)

AA => => FF[[BB]]
case class AncientRecord(
age: String,
name: String,
rateOfIncome: String
)
case class Citizen(
age: Int,
name: String,
rateOfIncome: Double
)
// fate of record not guaranteed, via F
def parseRecord(rec: AncientRecord): F[Citizen]

OptionOption[_][_]
def parseAge(s: String): Option[Int]
def parseName(s: String): Option[String]
def parseRate(s: String): Option[Double]

FOR COMPREHENSIONFOR COMPREHENSION
def parseRecord(rec: AncientRecord): Option[Citizen] =
for {
age <- parseAge(rec.age)
name <- parseName(rec.name)
rate <- parseRate(rec.rate)
} yield Citizen(age, name, rate)

PARSING WITH OPTIONPARSING WITH OPTION
for {
age <- parseAge(rec.age) // Some[Int]
name <- parseName(rec.name) // Some[String]
rate <- parseRate(rec.rate) // Some[Double]
} yield
Citizen(age, name, rate) // Some[Citizen]

NOT ALL AVAILABLENOT ALL AVAILABLE
for {
age <- parseAge(rec.age) // Some[Int]
name <- parseName(rec.name) // None
rate <- parseRate(rec.rate) // (does not run)
} yield
Citizen(age, name, rate) // None

FIRST ERROR WINSFIRST ERROR WINS

TryTry[_][_]
def parseAge(s: String): Try[Int]
def parseName(s: String): Try[String]
def parseRate(s: String): Try[Double]

PARSING WITH TRYPARSING WITH TRY
def parseRecord(rec: AncientRecord): Try[Citizen] =
for {
age <- parseAge(rec.age) // Success[Int]
name <- parseName(rec.name) // Failure[String]
} yield
Citizen(age, name, rate) // Failure[Citizen]

Name Happy path Sad path
Option[_] Some[_] None
Try[_] Success[_] Failure[_]

ERROR ADTERROR ADT
sealed trait ParsingErr
case class UnreadableAge (s: String) extends ParsingErr
case class UnreadableName(s: String) extends ParsingErr
case class UnreadableRate(s: String) extends ParsingErr

EitherEither[_, _][_, _]
def parseAge(s: String): Either[UnreadableAge, Int]
def parseName(s: String): Either[UnreadableName, String]
def parseRate(s: String): Either[UnreadableRate, Double]

PARSING WITH EITHERPARSING WITH EITHER
def parseRecord(rec: AncientRecord): Either[ParsingErr, Citizen] =
for {
age <- parseAge(rec.age) // Right[UnreadableAge, Int]
name <- parseName(rec.name) // Left[UnreadableName, String]
} yield
Citizen(age, name, rate) // Left[ParsingErr, Citizen]

Either[E, _] Right[E, _] Left[E, _]

def parseRecord(rec: AncientRecord): Either[ParsingErr, Citizen]

sealed trait HttpResponse
case class HttpSucccess (s: String) extends HttpResponse
case class HttpClientError(s: String) extends HttpResponse
def present(res: Either[ParsingErr, Citizen]): HttpResponse

sealed trait HttpResponse
case class HttpSucccess (s: String) extends HttpResponse
case class HttpClientError(s: String) extends HttpResponse
def present(res: Either[ParsingErr, Citizen]): HttpResponse
def httpError(err: ParsingError): HttpClientError
def httpSuccess(c: Citizen): HttpSucccess

USINGUSING .fold().fold()
val res: Either[ParsingErr, Citizen] = ???
val httpResponse: HttpResponse =
res
.fold(httpError, httpSuccess)

USINGUSING .leftMap().leftMap()
val res: Either[ParsingErr, Citizen] = ???
res // Either[ParsingErr, Citizen]
.map(httpSuccess) // Either[ParsingErr, HttpSucccess]
.leftMap(httpError) // Either[HttpClientError, HttpSucccess]
.merge // HttpResponse

EitherEither[[EE, , AA]] BIFUNCTORBIFUNCTOR
Can map over E
Can map over A

POSTFIX SYNTAXPOSTFIX SYNTAX
import cats.implicits._
def luckyNumber: Either[String, Int] =
if (Random.nextBoolean)
42.asRight
else
"You are unlucky.".asLeft

IOIO[_][_]
def parseAge(s: String): IO[Int]
def parseName(s: String): IO[String]
def parseRate(s: String): IO[Double]

PARSING WITH IOPARSING WITH IO
def parseRecord(rec: AncientRecord): IO[Citizen] =
for {
age <- parseAge(rec.age) // successful
name <- parseName(rec.name) // error "raised"
} yield
Citizen(age, name, rate) // error raised in step 2

IO[_] IO[_] IO[_]

MONAD TYPECLASSMONAD TYPECLASS
import cats._
import cats.effect._
import scala.util._
type EitherParsingErr[A] = Either[ParsingErr, A]
implicitly[Monad[Option]]
implicitly[Monad[Try]]
implicitly[Monad[EitherParsingErr]]
implicitly[Monad[IO]]

ERROR CHANNELSERROR CHANNELS
F[A] shape Payload Error channel
Try[A] A Throwable
IO[A] A Throwable
Either[E, A] A E

MONADERROR TYPECLASSMONADERROR TYPECLASS
import cats._
import scala.util._
implicitly[MonadError[Try, Throwable]]
implicitly[MonadError[IO, Throwable]]
implicitly[MonadError[EitherParsingErr, ParsingErr]]

TYPECLASS TABLETYPECLASS TABLE
Functor[_]
Applicative[_]
Monad[_] MonadError[_, _]

PARSING WITH EITHERPARSING WITH EITHER
def parseRecord(rec: AncientRecord): Either[ParsingErr, Citizen] =
for {
age <- parseAge(rec.age) // Left[ParsingErr, Int]
name <- parseName(rec.name) // (does not run)
} yield
Citizen(age, name, rate) // only reports first error

GIVEN:GIVEN:
EitherEither[[EE, , AA]]

GIVEN:GIVEN:
WANT:WANT:
EitherEither[[ListList[[EE], ], AA]]

MONADS START WITH:MONADS START WITH:

MONADS START WITH:MONADS START WITH:
MONADS END WITH:MONADS END WITH:

ValidatedValidated[_, _][_, _]

ValidatedValidated[_, _][_, _]
def parseAge(s: String): Validated[UnreadableAge, Int]
def parseName(s: String): Validated[UnreadableName, String]
def parseRate(s: String): Validated[UnreadableRate, Double]

PARSING WITH VALIDATED???PARSING WITH VALIDATED???
def parseRecord(rec: AncientRecord): ???[Citizen] =
for {
age <- ??? // ???
name <- ??? // ???
rate <- ??? // ???
} yield
Citizen(age, name, rate) // ???

ACCUMULATIONACCUMULATION
def combine(
v1: Validated[ParsingErr, Int],
v2: Validated[ParsingErr, String]):
Validated[List[ParsingError], (Int, String)] =
(v1, v2) match {
case (Valid(n), Valid(s)) => Valid((n, s))
case (Invalid(err), Valid(s)) => Invalid(List(err))
case (Valid(n), Invalid(err)) => Invalid(List(err))
case (Invalid(e1), Invalid(e2)) => Invalid(List(e1, e2))
}

ValidatedValidated[[EE, , AA]]

VALIDATED AND EITHERVALIDATED AND EITHER
Used for capturing errors as data
"Exclusive disjunction"
(payload A or error E but never both)
Bifunctor over A and E

IO[_] IO[_] IO[_]
Validated[E, _] Valid[_] Invalid[E]

ERROR CHANNELSERROR CHANNELS
F[A] shape Payload Error channel
Try[A] A Throwable
IO[A] A Throwable
Either[E, A] A E
Validated[E, A] A E

APPLICATIVEERROR TYPECLASSAPPLICATIVEERROR TYPECLASS
import cats._
type ValidatedParsingErr[A] = Validated[ParsingErr, A]
implicitly[ApplicativeError[IO, Throwable]]
implicitly[ApplicativeError[EitherParsingErr, ParsingErr]]
implicitly[ApplicativeError[ValidatedParsingErr, ParsingErr]]
// does not compile
implicitly[MonadError[ValidatedParsingErr, ParsingErr]]

TYPECLASS TABLETYPECLASS TABLE
Functor[_]
Applicative[_] ApplicativeError[_, _]
Monad[_] MonadError[_, _]

def luckyNumber: Validated[String, Int] =
42.valid
else
"You are unlucky.".invalid


ValidatedValidated[[ListList[[EE], ], AA]]

ValidatedValidated[[ListList[_], _][_], _]
import cats.data._
def parseAge(s: String):
Validated[List[UnreadableAge], Int]
def parseName(s: String):
Validated[List[UnreadableName], String]
def parseRate(s: String):
Validated[List[UnreadableRate], Double]

ACCUMULATIONACCUMULATION
def combine(
v1: Validated[List[ParsingErr], Int],
v2: Validated[List[ParsingErr], String]):
Validated[List[ParsingError], (Int, String)] =
(v1, v2) match {
case (Invalid(err), Valid(s)) => Invalid(err)
case (Valid(n), Invalid(err)) => Invalid(err)
case (Invalid(e1), Invalid(e2)) => Invalid(e1 ::: e2)
}

WITH SEMIGROUPWITH SEMIGROUP
def combine[F : Semigroup](
v1: Validated[F[ParsingErr], Int],
v2: Validated[F[ParsingErr], String]):
Validated[F[ParsingError], (Int, String)] =
(v1, v2) match {
case (Invalid(err), Valid(s)) => Invalid(err)
case (Valid(n), Invalid(err)) => Invalid(err)
// powered by Semigroup[F]
case (Invalid(e1), Invalid(e2)) => Invalid(e1 |+| e2)
}

ValidatedValidated[[FF[[EE], ], AA]]
FF : : SemigroupSemigroup

TUPLE SYNTAXTUPLE SYNTAX
import cats._
import cats.data._
val res: Validated[List[ParsingErr], Citizen] =
(
parseAge(rec.age),
parseName(rec.name),
parseRate(rec.rate)
)
.mapN(Citizen)

USINGUSING .fold().fold()
val res: Validated[List[ParsingErr], Citizen] = ???
def httpErrors(errs: List[ParsingError]): HttpClientError
res
.fold(httpErrors, httpSuccess)

USINGUSING .leftMap().leftMap()
val res: Validated[List[ParsingErr], Citizen] = ???
def httpErrors(errs: List[ParsingError]): HttpClientError
res // Validated[List[ParsingErr], Citizen]
.map(httpSuccess) // Validated[List[ParsingErr], HttpSucccess]
.leftMap(httpErrors) // Validated[HttpClientError, HttpSucccess]
.merge // HttpResponse

ACCUMULATEDACCUMULATED
ERRORSERRORS

Modeling dependent validations
Unwrapping envelopes or decoding

Modeling dependent validations
Unwrapping envelopes or decoding
Introducing gates/dependency onto independent
validations
Save on expensive computation

ACCUMULATED ERRORSACCUMULATED ERRORS

Modeling independent validations
Field-based structures (maps, records, objects)

Minimize round-trips

Minimize round-trips
Embracing parallelization

ListList[_][_]
import cats._
import cats.data._
Validated[List[UnreadableAge], Int]
Validated[List[UnreadableName], String]
Validated[List[UnreadableRate], Double]
implicitly[Semigroup[List]]

NonEmptyListNonEmptyList[_][_]
import cats._
import cats.data._
Validated[NonEmptyList[UnreadableAge], Int]
Validated[NonEmptyList[UnreadableName], String]
Validated[NonEmptyList[UnreadableRate], Double]
implicitly[Semigroup[NonEmptyList]]

import cats.data._
def luckyNumber: Validated[NonEmptyList[String], Int] =
42.validNel
else
"You are unlucky.".invalidNel

APPENDINGAPPENDING
List(1, 2, 3) ++ List(4)

APPENDING FREQUENTLYAPPENDING FREQUENTLY
List(1, 2, 3) ++ List(4)
List(1, 2, 3, 4) ++ List(5)
List(1, 2, 3, 4, 5) ++ List(8, 9)
List(1, 2) ++ List(3, 4) ++ List(8, 9)

LIST ACCUMULATIONLIST ACCUMULATION
// read from left to right
List(1, 2, 3, 4, 5) ++ List(8, 9)

LIST ACCUMULATION (A)LIST ACCUMULATION (A)
// via repeated traversals
List(1, 2, 3, 4, 5) ++ List(8, 9)
List(1, 2, 3, 4) ++ List(5, 8, 9)
List(1, 2, 3) ++ List(4, 5, 8, 9)
List(1, 2) ++ List(3, 4, 5, 8, 9)
List(1) ++ List(2, 3, 4, 5, 8, 9)
List(1, 2, 3, 4, 5, 8, 9)

LIST ACCUMULATION (B)LIST ACCUMULATION (B)
// via reverse list, built using fast prepend
List(1, 2, 3, 4) ++ ReverseList()
List(2, 3, 4) ++ ReverseList(1)
List(3, 4) ++ ReverseList(2, 1)
List(4) ++ ReverseList(3, 2, 1)
List() ++ ReverseList(4, 3, 2, 1)

SEQUENCESSEQUENCES
List(1, 2, 3, 4)
Seq(5)
Vector(6, 7, 8)
Nested(
List(9),
Array(10, 11, 12))

CHAINCHAINED TOGETHERED TOGETHER
Chain(
List(1, 2, 3, 4),
Seq(5),
Vector(6, 7, 8),
Chain(
List(9),
Array(10, 11, 12)))

ChainChain[_][_]
Validated[Chain[UnreadableAge], Int]
Validated[Chain[UnreadableName], String]
Validated[Chain[UnreadableRate], Double]
implicitly[Semigroup[Chain]]

NonEmptyChainNonEmptyChain[_][_]
Validated[NonEmptyChain[UnreadableAge], Int]
Validated[NonEmptyChain[UnreadableName], String]
Validated[NonEmptyChain[UnreadableRate], Double]
implicitly[Semigroup[NonEmptyChain]]

SEMIGROUPSEMIGROUP COLLECTIONSCOLLECTIONS
Name Provider Empty? Accumulation
performance
List Scala ✅ O(n^2)
NonEmptyList ❌ O(n^2)
Chain ✅ constant
NonEmptyChain ❌ constant

def luckyNumber: Validated[NonEmptyChain[String], Int] =
42.validNec
else
"You are unlucky.".invalidNec

ALGEBRASALGEBRAS
trait ParserAlg[F[_]] {
def parseAge(s: String): F[Int]
def parseName(s: String): F[String]
def parseRate(s: String): F[Double]
}

TYPECLASS METHODSTYPECLASS METHODS
class BadParser[F[_]](implicit F: ApplicativeError[F, Throwable]) {
def parseAge(s: String): F[Int] =
// raised, not thrown
F.raiseError(new Exception("what a world"))
}

NESTEDNESTED FF ANDAND GG
trait FancyParserAlg[F[_], G[_]] {
def parseAge(s: String): F[G[Int]]
def parseName(s: String): F[G[String]]
def parseRate(s: String): F[G[Double]]
}

FF WITH TRANSFORMERWITH TRANSFORMER GTGT
trait FancyParserAlg[F[_]] {
def parseAge(s: String): GT[F, Int]
def parseName(s: String): GT[F, String]
def parseRate(s: String): GT[F, Double]
}

FURTHER READINGFURTHER READING
Contains Chain and Validated, from the Cats
microsite
Cats Data Types

JOBS.DISNEYCAREERS.COMJOBS.DISNEYCAREERS.COM

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Functional Error Handling with Cats

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