Logging without Tradeoffs

This is a little detour into the world of logging in functional
programming. In this talk we will discuss journalling, timeliness,
memory & resource usage; laugh at tutorials that scream "WRITER" then
demonstrate it with a buggy piece of code that should never go to
production; and finally build up a small library of code for logging
that isn't horribly crippled.

This was presented at ScalaSyd, May 2015.

Transcript

1. Logging Without Tradeoffs
   

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2. define: logging
   

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3. Logging is the cutting, skidding, on-site
   processing, and loading of trees or logs onto
   trucks or skeleton cars. In forestry, the term
   logging is sometimes used in a narrow sense
   concerning the logistics of moving wood from
   the stump to somewhere outside the forest,
   usually a sawmill or a lumber yard.
   Wikipedia - On Logging!
   (i.e. can’t be wrong)
   (disclaimer: could actually be wrong)
   

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4. … logistics of moving wood
   from the stump to somewhere
   outside the forest, usually a
   sawmill or a lumber yard.
   Wikipedia - On Logging!
   (i.e. can’t be wrong)
   (disclaimer: could actually be wrong)
   

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5. Sawmill
   

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6. Sawmill
   Lumber Yard
   

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7. how?
   

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8. &1
   

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9. an example
   

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10. View Slide

11. View Slide

12. !
    1 case class Stock(name: String)
    !
    !
    !
    !
    !
    !
    !
    !
    !
    !
    !
    !
    

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13. !
    1 case class Stock(name: String)
    2
    3 sealed trait Order
    4 case class Buy(stock: Stock, qty: Long) extends Order
    5 case class Sell(stock: Stock, qty: Long) extends Order
    6 case object Hold extends Order
    !
    !
    !
    !
    !
    !
    !
    !
    

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14. !
    1 case class Stock(name: String)
    2
    3 sealed trait Order
    4 case class Buy(stock: Stock, qty: Long) extends Order
    5 case class Sell(stock: Stock, qty: Long) extends Order
    6 case object Hold extends Order
    7
    8 case class Tick(time: Timestamp, stock: Stock, cents: Long)
    !
    !
    !
    !
    !
    !
    

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15. !
    1 case class Stock(name: String)
    2
    3 sealed trait Order
    4 case class Buy(stock: Stock, qty: Long) extends Order
    5 case class Sell(stock: Stock, qty: Long) extends Order
    6 case object Hold extends Order
    7
    8 case class Tick(time: Timestamp, stock: Stock, cents: Long)
    9
    10 case class Quanity(stock: String, qty: Long)
    \
    !
    !
    !
    

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16. !
    1 case class Stock(name: String)
    2
    3 sealed trait Order
    4 case class Buy(stock: Stock, qty: Long) extends Order
    5 case class Sell(stock: Stock, qty: Long) extends Order
    6 case object Hold extends Order
    7
    8 case class Tick(time: Timestamp, stock: Stock, cents: Long)
    9
    10 case class Quanity(stock: String, qty: Long)
    11
    12 case class Budget(cash: Long, stocks: List[Quanitity])
    !
    !
    

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17. !
    1 case class Stock(name: String)
    2
    3 sealed trait Order
    4 case class Buy(stock: Stock, qty: Long) extends Order
    5 case class Sell(stock: Stock, qty: Long) extends Order
    6 case object Hold extends Order
    7
    8 case class Tick(time: Timestamp, stock: Stock, cents: Long)
    9
    10 case class Quanity(stock: String, qty: Long)
    11
    12 case class Budget(cash: Long, stocks: List[Quantity])
    13
    14 case class Decision(model: Model, budget: Budget, order: Order)
    

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18. 1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4
    5 case Some(ticks) =>
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20 }
    

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19. 1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4 Disconnected(exchange).left
    5 case Some(ticks) =>
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20 }
    

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20. 1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4 Disconnected(exchange).left
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20 }
    

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21. 1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4 Disconnected(exchange).left
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9
    10 case None =>
    11
    12
    13
    14
    15
    16
    17
    18
    19 }
    20 }
    

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22. 1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4 Disconnected(exchange).left
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 ProgrammersTryingToLoseYourMoney(decision, guard).left
    10 case None =>
    11
    12
    13
    14
    15
    16
    17
    18
    19 }
    20 }
    

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23. 1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4 Disconnected(exchange).left
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 ProgrammersTryingToLoseYourMoney(decision, guard).left
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13
    14 case TradeRejected =>
    15
    16 case TradeAccepted(budget) =>
    17
    18 }
    19 }
    20 }
    

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24. 1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4 Disconnected(exchange).left
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 ProgrammersTryingToLoseYourMoney(decision, guard).left
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13 BannedFor(decision, reason).left
    14 case TradeRejected =>
    15
    16 case TradeAccepted(budget) =>
    17
    18 }
    19 }
    20 }
    

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25. 1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4 Disconnected(exchange).left
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 ProgrammersTryingToLoseYourMoney(decision, guard).left
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13 BannedFor(decision, reason).left
    14 case TradeRejected =>
    15 bot(exchange, decision.model, decision.budget)
    16 case TradeAccepted(budget) =>
    17
    18 }
    19 }
    20 }
    

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26. 1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4 Disconnected(exchange).left
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 ProgrammersTryingToLoseYourMoney(decision, guard).left
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13 BannedFor(decision, reason).left
    14 case TradeRejected =>
    15 bot(exchange, decision.model, decision.budget)
    16 case TradeAccepted(budget) =>
    17 bot(exchange, decision.model, budget)
    18 }
    19 }
    20 }
    

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27. !
    1 def decide(seed: Model, ticks: List[Tick], budget: Budget): Decision = {
    2
    3
    4
    5
    6
    7
    8
    9
    10 }
    

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28. !
    1 def decide(seed: Model, ticks: List[Tick], budget: Budget): Decision = {
    2 val model = seed.update(ticks, budget)
    3
    4
    5
    6
    7
    8
    9
    10 }
    

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29. !
    1 def decide(seed: Model, ticks: List[Tick], budget: Budget): Decision = {
    2 val model = seed.update(ticks, budget)
    3 val predictions = model.predictions
    4
    5
    6
    7
    8
    9
    10 }
    

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30. !
    1 def decide(seed: Model, ticks: List[Tick], budget: Budget): Decision = {
    2 val model = seed.update(ticks, budget)
    3 val predictions = model.predictions
    4 val offer =
    5 predictions
    6 .sortBy(p => -1 * p.confidence * p.score)
    7 .map(_.offer)
    8 .head
    9
    10 }
    

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31. !
    1 def decide(seed: Model, ticks: List[Tick], budget: Budget): Decision = {
    2 val model = seed.update(ticks, budget)
    3 val predictions = model.predictions
    4 val offer =
    5 predictions
    6 .sortBy(p => -1 * p.confidence * p.score)
    7 .map(_.offer)
    8 .head
    9 Decision(model, budget, offer)
    10 }
    

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32. fp much?
    

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33. !
    1 def bot(exchange: Exchange, model: Model, budget: Budget): Error \/ Unit =
    2 exchange.next match {
    3 case None =>
    4 Disconnected(exchange).left
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 ProgrammersTryingToLoseYourMoney(decision, guard).left
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13 BannedFor(decision, reason).left
    14 case TradeRejected =>
    15 bot(exchange, decision.model, decision.budget)
    16 case TradeAccepted(budget) =>
    17 bot(exchange, decision.model, budget)
    18 }
    19 }
    20 }
    

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34. !
    1 def bot(exchange: Exchange, model: Model, budget: Budget): EitherT[IO, Error, Unit] =
    2 exchange.next.flatMap({
    3 case None =>
    4 EitherT.left(Diconnected(exchange))
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 EitherT.left(ProgrammersTryingToLoseYourMoney(decision, guard))
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13 EitherT.left(BannedFor(decision, reason))
    14 case TradeRejected =>
    15 bot(exchange, decision.model, decision.budget)
    16 case TradeAccepted(budget) =>
    17 bot(exchange, decision.model, budget)
    18 }
    19 }
    20 })
    

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35. View Slide

36. !
    Error \/ Unit
    !
    EitherT[IO, Error, Unit]
    

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37. !
    Error \/ Unit
    !
    EitherT[IO, Error, Unit]
    !
    IO[Error \/ Unit]
    

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38. so logging…
    

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39. !
    1 def bot(exchange: Exchange, model: Model, budget: Budget): EitherT[IO, Error, Unit] =
    2 exchange.next.flatMap({
    3 case None =>
    4 EitherT.left(Diconnected(exchange))
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 EitherT.left(ProgrammersTryingToLoseYourMoney(decision, guard))
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13 EitherT.left(BannedFor(decision, reason))
    14 case TradeRejected =>
    15 bot(exchange, decision.model, decision.budget)
    16 case TradeAccepted(budget) =>
    17 bot(exchange, decision.model, budget)
    18 }
    19 }
    20 })
    

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40. !
    1 def bot(exchange: Exchange, model: Model, budget: Budget): EitherT[IO, Error, Unit] =
    2 exchange.next.flatMap({
    3 case None =>
    4 EitherT.left(Diconnected(exchange))
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 EitherT.left(ProgrammersTryingToLoseYourMoney(decision, guard))
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13 EitherT.left(BannedFor(decision, reason))
    14 case TradeRejected =>
    15 bot(exchange, decision.model, decision.budget)
    16 case TradeAccepted(budget) =>
    17 bot(exchange, decision.model, budget)
    18 }
    19 }
    20 })
    !
    avoid it
    

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41. !
    1 sealed trait Error
    2
    3 case class BannedFor(
    4 decision: Decision
    5 , reason: Reason
    6 ) extends Error
    7
    8 case object Disconnected
    9 extends Error
    10
    11 case class ProgrammersTryingToLoseYourMoney(
    12 decision: Decision
    13 , check: Check
    14 ) extends Error
    !
    

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42. when you
    can’t?
    

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43. View Slide

44. (,)
    

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45. !
    1 def decide(seed: Model, ticks: List[Tick], budget: Budget): Decision = {
    2 val model = seed.update(ticks, budget)
    3 val predictions = model.predictions
    4 val offer =
    5 predictions
    6 .sortBy(p => -1 * p.confidence * p.score)
    7 .map(_.offer)
    8 .head
    9 Decision(model, budget, offer)
    10 }
    

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46. !
    1 def decide(s: Model, t: List[Tick], b: Budget): Writer[Log, Decision] = {
    2 val model = seed.update(ticks, budget)
    3 val predictions = model.predictions
    4 val offer =
    5 predictions
    6 .sortBy(p => -1 * p.confidence * p.score)
    7 .map(_.offer)
    8 .head
    9 Decision(model, budget, offer) 10 }
    

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47. !
    1 def bot(exchange: Exchange, model: Model, budget: Budget): EitherT[IO, Error, Unit] =
    2 exchange.next.flatMap({
    3 case None =>
    4 EitherT.left(Diconnected(exchange))
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 EitherT.left(ProgrammersTryingToLoseYourMoney(decision, guard))
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13 EitherT.left(BannedFor(decision, reason))
    14 case TradeRejected =>
    15 bot(exchange, decision.model, decision.budget)
    16 case TradeAccepted(budget) =>
    17 bot(exchange, decision.model, budget)
    18 }
    19 }
    20 })
    

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48. !
    1 def bot(exchange: Exchange, model: Model, budget: Budget): EitherT[IO, Error, Unit] =
    2 exchange.next.flatMap({
    3 case None =>
    4 EitherT.left(Diconnected(exchange))
    5 case Some(ticks) =>
    6 val decision = decide(model, ticks, budget)
    7 check(decision) match {
    8 case Some(guard) =>
    9 EitherT.left(ProgrammersTryingToLoseYourMoney(decision, guard))
    10 case None =>
    11 exchange.trade(decision.order, decision.budget) match {
    12 case Banned(reason) =>
    13 EitherT.left(BannedFor(decision, reason))
    14 case TradeRejected =>
    15 bot(exchange, decision.model, decision.budget)
    16 case TradeAccepted(budget) =>
    17 bot(exchange, decision.model, budget)
    18 }
    19 }
    20 })
    !
    fail at it
    

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49. 2 def bot(exchange: Exchange, model: Model, budget: Budget): EitherT[IO, Error, Unit] =
    3 exchange.next.flatMap({
    4 case None =>
    5 EitherT.left(Diconnected(exchange))
    6 case Some(ticks) =>
    7 val decision = decide(model, ticks, budget)
    8 println("got ticks: " + ticks.size)
    9 check(decision) match {
    10 case Some(guard) =>
    11 EitherT.left(ProgrammersTryingToLoseYourMoney(decision, guard))
    12 case None =>
    13 exchange.trade(decision.order, decision.budget) match {
    14 case Banned(reason) =>
    15 EitherT.left(BannedFor(decision, reason))
    16 case TradeRejected =>
    17 println("trade-rejected: " + decision)
    18 bot(exchange, decision.model, decision.budget)
    19 case TradeAccepted(budget) =>
    20 bot(exchange, decision.model, budget)
    21 }
    22 }
    23 })
    !
    fail at it
    

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50. 2 def bot(exchange: Exchange, model: Model, budget: Budget): EitherT[IO, Error, Unit] =
    3 exchange.next.flatMap({
    4 case None =>
    5 EitherT.left(Diconnected(exchange))
    6 case Some(ticks) =>
    7 val decision = decide(model, ticks, budget)
    8 putStrLn("got ticks: " + ticks.size).liftIO >> (
    9 check(decision) match {
    10 case Some(guard) =>
    11 EitherT.left(ProgrammersTryingToLoseYourMoney(decision, guard))
    12 case None =>
    13 exchange.trade(decision.order, decision.budget) match {
    14 case Banned(reason) =>
    15 EitherT.left(BannedFor(decision, reason))
    16 case TradeRejected =>
    17 putStrLn("trade-rejected: " + decision).liftIO >>
    18 bot(exchange, decision.model, decision.budget)
    19 case TradeAccepted(budget) =>
    20 bot(exchange, decision.model, budget)
    21 }
    22 })
    23 })
    !
    fail at it (again)
    

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51. after some time…
    we realise the need to
    pass arguments to
    functions
    

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52. and pass functions as
    arguments…
    

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53. 3 case class Trace(trace: String => IO[Unit])
    !
    

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54. !
    def bot(
    e: Exchange
    , m: Model
    , b: Budget
    , t: Trace
    ): EitherT[IO, Error, Unit]
    

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55. one weird trick…
    

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56. !
    def bot(
    e: Exchange
    , m: Model
    , b: Budget
    , t: Trace
    ): EitherT[IO, Error, Unit]
    

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57. !
    type App[A] = ReaderT[EitherT[IO, _], Trace[IO], A]
    !
    def bot(
    e: Exchange
    , m: Model
    , b: Budget
    ): App[IO, Error, Unit]
    

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58. but really…
    passing arguments
    is cool
    

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59. !
    def bot(
    e: Exchange
    , m: Model
    , b: Budget
    , t: Trace
    ): EitherT[IO, Error, Unit]
    

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60. 2 def bot(e: Exchange, m: Model, b: Budget, t: Trace): EitherT[IO, Error, Unit]
    3 exchange.next.flatMap({
    4 case None =>
    5 EitherT.left(Diconnected(exchange))
    6 case Some(ticks) =>
    7 val decision = decide(model, ticks, budget)
    8 t.trace(“got ticks: " + ticks.size).liftIO >> (
    9 check(decision) match {
    10 case Some(guard) =>
    11 EitherT.left(ProgrammersTryingToLoseYourMoney(decision, guard))
    12 case None =>
    13 exchange.trade(decision.order, decision.budget) match {
    14 case Banned(reason) =>
    15 EitherT.left(BannedFor(decision, reason))
    16 case TradeRejected =>
    17 t.trace(“trade-rejected: " + decision).liftIO >>
    18 bot(exchange, decision.model, decision.budget)
    19 case TradeAccepted(budget) =>
    20 bot(exchange, decision.model, budget)
    21 }
    22 })
    23 })
    

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61. this is what most
    people reach after
    doing fp (poorly) for a
    while
    

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62. and this is also
    the start of my actual
    talk …
    

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63. !
    Error \/ Unit
    !
    EitherT[IO, Error, Unit]
    !
    IO[Error \/ Unit]
    

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64. !
    Error \/ Unit
    !
    EitherT[F, Error, Unit]
    !
    F[Error \/ Unit]
    

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65. !
    def bot[F[_]: Monad](
    e: Exchange[F]
    , m: Model
    , b: Budget
    ): EitherT[F, Error, Unit]
    !
    bot[IO](
    real
    , model
    , budget
    ): EitherT[IO, Error, Unit]
    !
    bot[Option](
    memory
    , model
    , budget
    ): EitherT[Option, Error, Unit]
    

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66. !
    def bot[F[_]: Monad](
    e: Exchange[F]
    , m: Model
    , b: Budget
    ): EitherT[F, Error, Unit]
    !
    bot[IO](
    real
    , model
    , budget
    ): IO[Error \/ Unit]
    !
    bot[Option](
    memory
    , model
    , budget
    ): Option[Error \/ Unit]
    

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67. 3 case class Trace(trace: String => IO[Unit])
    !
    

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68. 3 case class Trace[F[_]](trace: String => F[Unit])
    !
    

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69. 3 case class Trace[F[_]](trace: String => F[Unit])
    !
    !
    avoid strings
    

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70. 3 case class Trace[F[_], A](trace: A => F[Unit])
    !
    !
    avoid strings
    

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71. 3 case class Trace[F[_], A](trace: A => F[Unit])
    !
    !
    seen this before?
    

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72. 3 type Trace[F[_], A] = ReaderT[F, A, Unit]
    !
    

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73. 1 sealed trait Log
    2
    3 case class Received(
    4 ticks: List[Tick]
    5 ) extends Error
    6
    7 case class Reject(
    8 decision: Decision
    9 )
    10
    11 case class Accepted(
    12 decision: Decision
    13 )
    14
    15 case class Prediction (
    16 predictions: NonEmptyList[Prediction]
    17 , selected: Offer
    18 )
    

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74. !
    def bot[F[_]: Monad](
    e: Exchange[F]
    , m: Model
    , b: Budget
    , t: Trace[F, Log]
    ): EitherT[F, Error, Unit]
    

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75. !
    def bot[F[_]: Monad](
    e: Exchange[F]
    , m: Model
    , b: Budget
    , t: Trace[F, Log]
    ): EitherT[F, Error, Unit]
    

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76. !
    def bot[F[_]: Monad](
    e: Exchange[F]
    , m: Model
    , b: Budget
    , t: Trace[EitherT[F, _, ?], Log]
    ): EitherT[F, Error, Unit]
    

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77. !
    def bot[F[_]: Monad](
    e: Exchange[F]
    , m: Model
    , b: Budget
    , t: Trace[EitherT[F, _, ?], Log]
    ): EitherT[F, Error, Unit]
    

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78. !
    def bot[F[_]: Monad](
    e: Exchange[F]
    , m: Model
    , b: Budget
    , t: Trace[EitherT[F, _, ?], Log]
    ): EitherT[F, Error, Unit]
    

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79. 2 type Trace[F[_], A] =
    3 ReaderT[F, A, Unit]
    4
    5 type Traceable[F[_], A, B] =
    6 ReaderT[F, Trace[F], A]
    

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80. !
    def bot[F[_]: Monad](
    e: Exchange[F]
    , m: Model
    , b: Budget
    ): Traceable[EitherT[F, Error, ?], Log, Unit]
    

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81. 2 type Trace[F[_], A] =
    3 ReaderT[F, A, Unit]
    4
    5 type Traceable[F[_], A, B] =
    6 ReaderT[F, Trace[F], A]
    7
    8 type Bot[F[_], A] =
    9 Traceable[F[_], Log, A]
    

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82. !
    def bot[F[_]: Monad](
    e: Exchange[F]
    , m: Model
    , b: Budget
    ): Bot[EitherT[F, Error, ?], Unit]
    

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83. !
    def decide(
    seed: Model
    , ticks: List[Tick]
    , budget: Budget
    ): Writer[Log, Decision]
    

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84. !
    def decide[F[_]: Monad](
    seed: Model
    , ticks: List[Tick]
    , budget: Budget
    ): Bot[F, Decision]
    

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85. 2 type Trace[F[_], A] =
    3 ReaderT[F, A, Unit]
    4
    5 type Traceable[F[_], A, B] =
    6 ReaderT[F, Trace[F], A]
    7
    8 type Bot[F[_], A] =
    9 Traceable[F[_], Log, A]
    

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86. View Slide

87. !
    1 def log[F[_]](message: A): Traceable[F, A, Unit] =
    2 ReaderT(t => t.trace(message))
    

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88. !
    1 def decide[F[_]](s: Model, t: List[Tick], b: Budget): Bot[F, Decision] = {
    2 val model = seed.update(ticks, budget)
    3 val predictions = model.predictions
    4 val offer =
    5 predictions
    6 .sortBy(p => -1 * p.confidence * p.score)
    7 .map(_.offer)
    8 .head
    9 log(Prediction(prediction, off)).as(
    10 Decision(model, budget, offer))
    11 }
    

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89. 2 def bot(e: Exchange, m: Model, b: Budget): Bot[EitherT[F, Error ?], Unit] =
    3 exchange.next.flatMap({
    4 case None =>
    5 EitherT.left(Diconnected(exchange)).lift
    6 case Some(ticks) =>
    7 val decision = decide(model, ticks, budget).lift
    8 log(Received(ticks)) >> (
    9 check(decision) match {
    10 case Some(guard) =>
    11 EitherT.left(ProgrammersTryingToLoseYourMoney(decision, guard)).lift
    12 case None =>
    13 exchange.trade(decision.order, decision.budget) match {
    14 case Banned(reason) =>
    15 EitherT.left(BannedFor(decision, reason)).lift
    16 case TradeRejected =>
    17 log(Rejected(decision)) >>
    18 bot(exchange, decision.model, decision.budget)
    19 case TradeAccepted(budget) =>
    20 bot(exchange, decision.model, budget)
    21 }
    22 })
    23 })
    

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90. 1
    2 def writer[F[_]: Applicative, A]: Trace[WriterT[F, Vector[A], ?]] =
    3 Trace(a => WriterT.tell(a))
    4
    5 def stream[A](out: java.io.PrintStream, show: A => String) : Trace[IO, A] =
    6 Trace(a => IO { out.println(show(a)) })
    7
    8 def out[A](show: A => String): Trace[IO, A] =
    9 stream(Console.out)
    10
    11 def err[A](show: A => String): Trace[IO, A] =
    12 stream(Console.err)
    13
    14 def noop[F[_]: Applicative, A]: Trace[F, A] =
    15 Trace(_ => ().pure[F])
    16
    17 def hoistEither[F[_], A](t: Trace[F, A]): Trace[EitherT[F, _, A]] =
    18 ???
    19
    20 def io[F[_], A](t: Trace[WriterT[F, Vector[A], ?], A]):Trace[IO, A] =
    22 ???
    

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91. 2 val d1: Decision =
    3 decide(seed, ticks, budget).run(noop[Id, Log])
    4
    5 val d2: (Vector[Log], Decision) =
    6 decide(seed, ticks, budget).run(writer[Log]).run
    7
    8 val d3: IO[Decision] =
    9 decide(seed, ticks, budget).run(out(render)).run
    

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92. 2 type Trace[F[_], A] =
    3 ReaderT[F, A, Unit]
    4
    5 type Traceable[F[_], A, B] =
    6 ReaderT[F, Trace[F], A]
    7
    8 type Bot[F[_], A] =
    9 Traceable[F[_], Log, A]
    

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93. 1
    2 def writer[F[_]: Applicative, A]: Trace[WriterT[F, Vector[A], ?]] =
    3 Trace(a => WriterT.tell(a))
    4
    5 def stream[A](out: java.io.PrintStream, show: A => String) : Trace[IO, A] =
    6 Trace(a => IO { out.println(show(a)) })
    7
    8 def out[A](show: A => String): Trace[IO, A] =
    9 stream(Console.out)
    10
    11 def err[A](show: A => String): Trace[IO, A] =
    12 stream(Console.err)
    13
    14 def noop[F[_]: Applicative, A]: Trace[F, A] =
    15 Trace(_ => ().pure[F])
    16
    17 def hoistEither[F[_], A](t: Trace[F, A]): Trace[EitherT[F, _, A]] =
    18 ???
    19
    20 def io[F[_], A](t: Trace[WriterT[F, Vector[A], ?], A]):Trace[IO, A] =
    22 ???
    

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94. 3 case class Trace[F[_], A](
    4 trace: A => F[Unit]
    5 , hoist: Trace[WriterT[F, Vector[A], ?], A]
    6 )
    

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95. View Slide