Sometimes we want to keep or collect errors for later examination. We’ll use Cats to help us pick the story we want to tell when handling errors; accumulated errors or first error wins. Monads will be included!

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

5. OPTIONOPTION[_][_]
TRYTRY[_][_]
EITHEREITHER[_, _][_, _]

6. FUNCTORFUNCTOR[_][_]
APPLICATIVEAPPLICATIVE[_][_]
MONADMONAD[_][_]

7. ERRORS ASERRORS AS DATADATA

10. 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

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

14. 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]

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

16. 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)

17. PARSING WITH OPTIONPARSING WITH OPTION
def parseRecord(rec: AncientRecord): Option[Citizen] =
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]

18. NOT ALL AVAILABLENOT ALL AVAILABLE
def parseRecord(rec: AncientRecord): Option[Citizen] =
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

19. FIRST ONE WINSFIRST ONE WINS

20. FIRST ERROR WINSFIRST ERROR WINS

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

25. PARSING WITH TRYPARSING WITH TRY
def parseRecord(rec: AncientRecord): Try[Citizen] =
for {
age <- parseAge(rec.age) // Success[Int]
name <- parseName(rec.name) // Failure[String]
rate <- parseRate(rec.rate) // (does not run)
} yield
Citizen(age, name, rate) // Failure[Citizen]

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

30. 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

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

33. 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]
rate <- parseRate(rec.rate) // (does not run)
} yield
Citizen(age, name, rate) // Left[ParsingErr, Citizen]

35. Name Happy path Sad path
Option[_] Some[_] None
Try[_] Success[_] Failure[_]
Either[E, _] Right[E, _] Left[E, _]

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

38. 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

39. 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
def httpError(err: ParsingError): HttpClientError
def httpSuccess(c: Citizen): HttpSucccess

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

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

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

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

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

47. PARSING WITH IOPARSING WITH IO
def parseRecord(rec: AncientRecord): IO[Citizen] =
for {
age <- parseAge(rec.age) // successful
name <- parseName(rec.name) // error "raised"
rate <- parseRate(rec.rate) // (does not run)
} yield
Citizen(age, name, rate) // error raised in step 2

48. Name Happy path Sad path
Option[_] Some[_] None
Try[_] Success[_] Failure[_]
Either[E, _] Right[E, _] Left[E, _]
IO[_] IO[_] IO[_]

50. 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]]

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

52. MONADERROR TYPECLASSMONADERROR TYPECLASS
import cats._
import cats.effect._
import scala.util._
type EitherParsingErr[A] = Either[ParsingErr, A]
implicitly[MonadError[Try, Throwable]]
implicitly[MonadError[IO, Throwable]]
implicitly[MonadError[EitherParsingErr, ParsingErr]]

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

57. 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)
rate <- parseRate(rec.rate) // (does not run)
} yield
Citizen(age, name, rate) // only reports first error

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

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

60. MONADS START WITH:MONADS START WITH:
EitherEither[[EE, , AA]]

61. MONADS START WITH:MONADS START WITH:
EitherEither[[EE, , AA]]
MONADS END WITH:MONADS END WITH:
EitherEither[[EE, , AA]]

64. ValidatedValidated[_, _][_, _]

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

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

67. 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))
}

68. EitherEither[[EE, , AA]]
ValidatedValidated[[EE, , AA]]

69. 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

70. Name Happy path Sad path
Option[_] Some[_] None
Try[_] Success[_] Failure[_]
Either[E, _] Right[E, _] Left[E, _]
IO[_] IO[_] IO[_]
Validated[E, _] Valid[_] Invalid[E]

71. 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

72. APPLICATIVEERROR TYPECLASSAPPLICATIVEERROR TYPECLASS
import cats._
import cats.effect._
type EitherParsingErr[A] = Either[ParsingErr, A]
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]]

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

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

77. ValidatedValidated[[EE, , AA]]
ValidatedValidated[[EE, , AA]]
ValidatedValidated[[EE, , AA]]
ValidatedValidated[[ListList[[EE], ], AA]]

78. EitherEither[[EE, , AA]]
EitherEither[[EE, , AA]]
EitherEither[[EE, , AA]]
EitherEither[[EE, , AA]]

79. ValidatedValidated[[ListList[[EE], ], AA]]
ValidatedValidated[[ListList[[EE], ], AA]]
ValidatedValidated[[ListList[[EE], ], AA]]
ValidatedValidated[[ListList[[EE], ], AA]]

80. 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]

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

82. 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 (Valid(n), Valid(s)) => Valid((n, s))
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)
}

83. ValidatedValidated[[EE, , AA]]

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

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

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

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

90. FIRST ERROR WINSFIRST ERROR WINS

91. ACCUMULATEDACCUMULATED
ERRORSERRORS

92. FIRST ERROR WINSFIRST ERROR WINS

93. FIRST ERROR WINSFIRST ERROR WINS
Modeling dependent validations
Unwrapping envelopes or decoding

94. FIRST ERROR WINSFIRST ERROR WINS
Modeling dependent validations
Unwrapping envelopes or decoding
Introducing gates/dependency onto independent
validations
Save on expensive computation

95. ACCUMULATED ERRORSACCUMULATED ERRORS

96. ACCUMULATED ERRORSACCUMULATED ERRORS
Modeling independent validations
Field-based structures (maps, records, objects)

97. ACCUMULATED ERRORSACCUMULATED ERRORS
Modeling independent validations
Field-based structures (maps, records, objects)
Minimize round-trips

98. ACCUMULATED ERRORSACCUMULATED ERRORS
Modeling independent validations
Field-based structures (maps, records, objects)
Minimize round-trips
Embracing parallelization

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

102. NonEmptyListNonEmptyList[_][_]
import cats._
import cats.data._
import cats.implicits._
def parseAge(s: String):
Validated[NonEmptyList[UnreadableAge], Int]
def parseName(s: String):
Validated[NonEmptyList[UnreadableName], String]
def parseRate(s: String):
Validated[NonEmptyList[UnreadableRate], Double]
implicitly[Semigroup[NonEmptyList]]

103. POSTFIX SYNTAXPOSTFIX SYNTAX
import cats.data._
import cats.implicits._
def luckyNumber: Validated[NonEmptyList[String], Int] =
if (Random.nextBoolean)
42.validNel
else
"You are unlucky.".invalidNel

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

106. 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)

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

109. 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)

110. 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)

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

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

113. ChainChain[_][_]
def parseAge(s: String):
Validated[Chain[UnreadableAge], Int]
def parseName(s: String):
Validated[Chain[UnreadableName], String]
def parseRate(s: String):
Validated[Chain[UnreadableRate], Double]
implicitly[Semigroup[Chain]]

114. NonEmptyChainNonEmptyChain[_][_]
def parseAge(s: String):
Validated[NonEmptyChain[UnreadableAge], Int]
def parseName(s: String):
Validated[NonEmptyChain[UnreadableName], String]
def parseRate(s: String):
Validated[NonEmptyChain[UnreadableRate], Double]
implicitly[Semigroup[NonEmptyChain]]

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

116. POSTFIX SYNTAXPOSTFIX SYNTAX
import cats.implicits._
def luckyNumber: Validated[NonEmptyChain[String], Int] =
if (Random.nextBoolean)
42.validNec
else
"You are unlucky.".invalidNec

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

119. 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"))
}

120. 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]]
}

121. 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]
}

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

123. JOBS.DISNEYCAREERS.COMJOBS.DISNEYCAREERS.COM

124. @@mark canlas nycmark canlas nyc