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cats in practice
੨ࢁΤϯδχΞษڧަྲྀձʙScalaษڧձʙ
ؔܕϥΠϒϥϦcatsͷΞυςΫ࣮ӡ༻Ͱͷ༻ྫ
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ञҪ ਸࢸ
- ౦େӃ ใཧ தୀ
- 2016/01- F@N Communicationsגࣜձࣾ
- Scala Engineer / Data Scientist
- Slack & Raspberry PiͰΤΞίϯ͚ͭͨΓ
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લఏͱ͍ͯ͠Δࣝ
- جຊతͳScalaͷࣝ
- ߏจ
- Option, List, Either, TryͳͲͷ
جຊతͳϞφυͷѻ͍ํ
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͞ͳ͍͜ͱ
- ਂ͍ؔܕϓϩάϥϛϯάͷ
- ϞφϞφͨ͜͠ͱͱ͔
- IOϞφυͱ͔
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What is cats?
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GitHub
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GitHub ελʔͪΐ͍
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catsͱ
- Scalaʹ͓͚ΔؔܕϥΠϒϥϦͷσϑΝΫτελϯμʔυ
- ҎԼͷΑ͏ͳؔܕ৭ͷڧ͍ϥΠϒϥϦ͕෦Ͱ
catsʹґଘ͍ͯ͠Δ
- circeʢJSONॲཧʣ
- doobieʢDBΞΫηεʣ
- finchʢHTTPϧʔςΟϯάʣ
- refinedʢมͷʹΑΔݫ֨ͳܕ੍ʣ
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հ͢Δͷ
- syntax.option
- syntax.either
- NonEmptyList
- sequence, traverse
- OptionT, EitherT
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syntax.option
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syntax.option._
// ී௨
val i: Option[Int] = Some(1)
val j: Option[Int] = None
——————————————————————————————
// cats
import cats.syntax.option._
val i: Option[Int] = 1.some
val j: Option[Int] = none
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syntax.either
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syntax.either._
val a2c : A => C
val b2c : B => C
val b2d : B => D
val either : Either[A, B]
either.leftMap(a2C) : Either[C, B]
either.bimap(a2c, b2d) : Either[C, D]
either.bifoldMap(a2c, b2c) : C
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“leftMap” in practice
// jsonจࣈྻΛܕAʹύʔε͢Δɻࣦഊ͢Δͱͦͷཧ༝Λॻ͍ͨ
String͕ฦͬͯ͘Δɻ
val either: Either[String, A] = Json.decode(str)
// StringͰҙຯ͕ᐆດͳͷͰɺErrorܕʹͯ͠Left͕Ͳ͏͍
͏ҙຯͳͷ͔໌֬ʹ͢Δɻ
either.leftMap(s => new CustomError(s))
: Either[CustomError, A]
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“bifoldMap” in practice
// ৴͍ͨ͠ࠂʢAdʣΛऔಘ͢Δɻ͠ͳ͍߹ɺԿނͳ͍ͷ͔
ͦͷཧ༝Λҙຯ͢ΔNoAdReason͕ฦͬͯ͘Δɻ
val result: Either[NoAdReason, Ad] = getAd
// ݁ՌΛϩάʹྲྀؔ͢ɻޭ͢Δͱtrueɻ
val logNoAdReason: NoAdReason => Boolean
val logAd : Ad => Boolean
// ϩάʹྲྀ͢
val succeeded = result.bifoldMap(logNoAdReason, logAd)
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NonEmptyList
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NonEmptyList (a.k.a “Nel”)
- ۭͰͳ͍͜ͱ͕อূ͞Ε͍ͯΔList
scala> import cats.syntax.list._
scala> List(1,2,3).toNel
res0: Option[cats.data.NonEmptyList[Int]] =
Some(NonEmptyList(1, 2, 3))
scala> List.empty[Int].toNel
res1: Option[cats.data.NonEmptyList[Int]] = None
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List vs NonEmptyList
val list : List[Int]
list.head : Int // unsafe
list.tail : List[Int] // unsafe
list.headOption : Option[Int] // safe
val nel : NonEmptyList[Int]
nel.head : Int // safe
nel.tail : List[Int] // safe
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“NonEmptyList” in practice
def getCandidateAds: List[Ad] // ৴Ͱ͖ΔࠂʢAdʣ
def getAdditionalInfo: Option[Info] // ࠷దͳࠂΛܾΊΔͨΊͷใ
def chooseBestAd(info: Info, ads: NonEmptyList[Ad]): Ad
for {
adNel <- getCandidateAds.toNel
info <- getAdditionalInfoɹ// ৴Ͱ͖Δࠂ͕ͳ͚ΕݺΕͳ͍
} yield chooseBestAd(info, adNel)
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sequence, traverse
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sequence, traverse
scala> import cats.implicits._
scala> (List(Some(1), Some(2)): List[Option[Int]]).sequence
res0: Option[List[Int]] = Some(List(1, 2))
scala> (List(Some(1), None): List[Option[Int]]).sequence
res1: Option[List[Int]] = None
scala> (Some(List(1,2,3)): Option[List[Int]]).sequence
res2: List[Option[Int]] = List(Some(1), Some(2), Some(3))
scala> (None: Option[List[Int]]).sequence
res3: List[Option[Int]] = List(None)
- 2ॏʹแ·ΕͨMonadͷॱ൪Λͻͬ͘Γฦ͢
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sequence, traverse
scala> import cats.implicits._
scala> (List(Some(1), Some(2)): List[Option[Int]]).sequence
res0: Option[List[Int]] = Some(List(1, 2))
scala> (List(Some(1), None): List[Option[Int]]).sequence
res1: Option[List[Int]] = None
scala> (Some(List(1,2,3)): Option[List[Int]]).sequence
res2: List[Option[Int]] = List(Some(1), Some(2), Some(3))
scala> (None: Option[List[Int]]).sequence
res3: List[Option[Int]] = List(None)
- 2ॏʹแ·ΕͨMonadͷॱ൪Λͻͬ͘Γฦ͢
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“sequence” in practice
// csvͷ֤ߦΛAʹมɻ1ߦͰࣦഊͨ͠ΒɺશମΛࣦഊʹ͢Δ
def parseCsv(
csv: List[String],
parse: String => Try[A],
): Try[List[A]] = {
val parsed: List[Try[A]] = csv.map(parse)
parsed.sequence
}
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ύοͱݟɺΑͦ͞͏͚ͩͲ…
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“sequence” in practice
// csvͷ֤ߦΛAʹมɻ1ߦͰࣦഊͨ͠ΒɺશମΛࣦഊʹ͢Δ
def parseCsv(
csv: List[String],
parse: String => Try[A],
): Try[List[A]] = {
val parsed: List[Try[A]] = csv.map(parse)
parsed.sequence
}
ߦͰࣦഊ͢Δ߹ͰɺશߦQBSTFͯ͠͠·͏
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͜͏͍͏߹traverseʂ
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“traverse” in practice
// csvͷ֤ߦΛAʹมɻ1ߦͰࣦഊͨ͠ΒɺશମΛࣦഊʹ͢Δ
def parseCsv(
csv: List[String],
parse: String => Try[A],
): Try[List[A]] = {
// ࣦഊͨ͠ߦ͕͋ͬͨΒͦͷ࣌Ͱऴྃ
csv.traverse(parse)
}
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OptionT, EitherT
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OptionT, EitherT
// Redis͔ΒURLΛऔಘ
def getFromRedis(key: String): Future[Option[String]]
// ͦͷURLʹHTTPΞΫηεͯ͠IDΛऔಘ
def getViaHttp(url: String): Future[Int]
// ͦͷIDͰAΛݟ͚ͭΔ
def findByIdInCache(id: Int): Option[A]
//※్தͰFuture͕ࣦഊ or Option͕NoneͩͬͨΒɺͦ͜Ͱॲཧऴྃ
- 2ॏMonadͷϘΠϥʔϓϨʔτΛݮΒ͢
- ҎԼͷΑ͏ͳྫΛߟ͑Δ
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OptionT, EitherT
.POBE͕ॏʹͳͬͯͨΓɺ
ҧ͏छྨͩͬͨΓ͍ͯ͜͠͠
// Redis͔ΒURLΛऔಘ
def getFromRedis(key: String): Future[Option[String]]
// ͦͷURLʹHTTPΞΫηεͯ͠IDΛऔಘ
def getViaHttp(url: String): Future[Int]
// ͦͷIDͰAΛݟ͚ͭΔ
def findByIdInCache(id: Int): Option[A]
//※్தͰFuture͕ࣦഊ or Option͕NoneͩͬͨΒɺͦ͜Ͱॲཧऴྃ
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// Redis͔ΒURLΛऔಘ
def getFromRedis(key: String): Future[Option[String]]
// ͦͷURLʹHTTPΞΫηεͯ͠IDΛऔಘ
def getViaHttp(url: String): Future[Int]
// ͦͷIDͰAΛݟ͚ͭΔ
def findByIdInCache(id: Int): Option[A]
for {
url <- getFromRedis(“key”)
id <- getViaHttp(url)
a <- findByIdInCache(id)
} yield a
.POBEͷछྨ͕ҧ͏ͷͰ
GPSࣜͰճ͢͜ͱͰ͖ͳ͍
OptionT, EitherT
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ͦΕ͕Ͱ͖ΔΜͰ͢ʂ
… ͦ͏ɺOptionTͳΒͶ
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import cats.data.OptionT
def getFromRedis(key: String): Future[Option[String]]
def getViaHttp(url: String): Future[Int]
def findByIdInCache(id: Int): Option[A]
val result: OptionT[Future, A] = for {
url: String <- OptionT(getFromRedis(“key”))
id : Int <- OptionT.liftF(getViaHttp(url))
a : A <- OptionT.fromOption(findByIdInCache(id))
} yield a
result.value // Future[Option[A]]
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import cats.data.OptionT
def getFromRedis(key: String): Future[Option[String]]
def getViaHttp(url: String): Future[Int]
def findByIdInCache(id: Int): Option[A]
val result: OptionT[Future, A] = for {
url: String <- OptionT(getFromRedis(“key”))
id : Int <- OptionT.liftF(getViaHttp(url))
a : A <- OptionT.fromOption(findByIdInCache(id))
} yield a
result.value // Future[Option[A]]
0QUJPO5ͷؔΛͬͯɺ
શ෦0QUJPO5<'VUVSF
">ܕʹม
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import cats.data.OptionT
def getFromRedis(key: String): Future[Option[String]]
def getViaHttp(url: String): Future[Int]
def findByIdInCache(id: Int): Option[A]
val result: OptionT[Future, A] = for {
url: String <- OptionT(getFromRedis(“key”))
id : Int <- OptionT.liftF(getViaHttp(url))
a : A <- OptionT.fromOption(findByIdInCache(id))
} yield a
result.value // Future[Option[A]]
࠷ޙʹWBMVFؔͰ
'VUVSF<0QUJPO<">ʹ͢
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import cats.data.EitherT
def getFromRedis(key: String): Future[Either[Error, String]]
def getViaHttp(url: String): Future[Int]
def findByIdInCache(id: Int): Either[Error, A]
val result: EitherT[Future, Error, A] = for {
url: String <- EitherT(getFromRedis(“key”))
id : Int <- EitherT.liftF(getViaHttp(url))
a : A <- EitherT.fromEither(findByIdInCache(id))
} yield a
result.value // Future[Either[Error, A]]
EitherT ͱ͍͏ͷ͋Γ·͢
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࠷ޙʹ
- ʮ͜ͷఔͷཧղͰcatsʹखΛग़͢ͳΜͯ…ʯ
ͱ͍͏ҙݟ͋Δͱࢥ͍·͢
- ·ͣͬͯΈΔ͜ͱେࣄʢʁʣ
- ͬͯΔʹͩΜͩΜཧղͯ͘͠Δ
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͋Γ͕ͱ͏͍͟͝·ͨ͠ʂ
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