scala/scala

A relaxed chat room about all things Scala (2 & 3 both). Beginner questions welcome. https://scala-lang.org/conduct/ applies

RoelofWobben

@RoelofWobben

How do I refractor the whole flatMap thing here :

sealed trait Expression{

  def eval: Sum[String, Double] = this match {

    case Addition(left, right) => left.eval flatMap { x => right.eval flatMap(y => Success(x + y))}
    case  Subtraction(left, right) => left.eval flatMap { x => right.eval flatMap(y => Success(x - y))}
}

Gavin Bisesi

@Daenyth

what monad is left/right/Success in?

RoelofWobben

@RoelofWobben

Sorry, monads are not explained yet. That will be in the cats book

RoelofWobben

@RoelofWobben

@Daenyth

Bastin Orar

@bastinOrar_twitter

I want a link to any beginner ML post with scala

Rob Norris

@tpolecat

@RoelofWobben assuming your result type acts like Either you can probably say for { x <- right.eval; y <- left.eval } yield x + y for example. That's probably where they are headed.

Gavin Bisesi

@Daenyth

@tpolecat I would have said that also except that he has a flatMap there on Success, so the yield is wrong

but that feels weird

@RoelofWobben what type are you calling flatMap on? Success is one of the members and left and right both have that type

Fabio Labella

@SystemFw

why do you find it weird?

Rob Norris

@tpolecat

I think that's like flatMap into pure … consider e.flatMap(a => Right(f(a))) which is e.map(f)

Fabio Labella

@SystemFw

yes

that's it

it's a map, so yield

Avremel Kaminetzky

@avremel

I have values in Future[Try[Option[Double]]] and Future[Option[Double]]. How can I add them? Should I be using Future.sequence?

RoelofWobben

@RoelofWobben

bad luck for me , yield is also not explained

Avremel Kaminetzky

@avremel

Or is there a way to use Future[Try[Option[Double]]] as Future[Option[Double]]?

RoelofWobben

@RoelofWobben

so there is no way I can refractor the this part out :

left.eval flatMap { x => right.eval flatMap(y

Gavin Bisesi

@Daenyth

@RoelofWobben

for {
  x <- left.eval
  y <- right.eval
} yield x + y

RoelofWobben

@RoelofWobben

oke, and I can put that into a function and call it

Fabio Labella

@SystemFw

I think the reason those things are not explained is that they are using the exercises to build the intuition you need to understand for/yield, once they do get there

RoelofWobben

@RoelofWobben

nope, in the other where I use it , it's x - y

so that code is not working

Fabio Labella

@SystemFw

it is

RoelofWobben

@RoelofWobben

@SystemFw for /yield is explained in the next chapter

Fabio Labella

@SystemFw

just pass an f: (Int, Int) => Int

and yield f(x, y)

exactly, so by doing this refactoring you are kinda "cheating"

RoelofWobben

@RoelofWobben

oke, then it let it be

I do not like cheating

Marcin Sokrates

@marcinsokrates_twitter

@avremel you should refactor to get rid of Future[Try[Option[Double]]] because it's redundant, Future.fromTry lets you skip the Try

RoelofWobben

@RoelofWobben

thanks all

time for the chapter about the for/yield and Option

Avremel Kaminetzky

@avremel

@marcinsokrates_twitter interesting. I was using toOption.flatten to remove the Try

  private def calculateShippingPriceRedo(cookie: String, country: Option[String], state: Option[String], shippingMethodId: Option[Int] = None): Future[Option[Double]] = {
    val countryWithDefault = country.getOrElse("US")

    shippingOptionsFull(cookie, country=countryWithDefault, state).map{ opt => opt.map{ rows =>
      shippingMethodId.flatMap{ id =>
          rows.find(_.shippingSpeedType === id ).orElse(rows.headOption).map(_.charge)
        }
      }.toOption.flatten
    }
  }

Derek Wickern

@dwickern

it depends what you want

Avremel Kaminetzky

@avremel

How would I use Future.fromTry?

Derek Wickern

@dwickern

do you want a successful future with None when the Try fails

Rob Norris

@tpolecat

Future[Try is usually wrong because Future already has the bulletproof behavior.

Derek Wickern

@dwickern

or a failed future

Avremel Kaminetzky

@avremel

Ye, it's someone's code, i don't want to refactor his code yet

Rob Norris

@tpolecat

So fto.flatMap(Future.fromTry) should give you Future[Option[Double]]

Avremel Kaminetzky

@avremel

Oh, thanks for that

And OptionT to add Future[Option[Double]], right?

Rob Norris

@tpolecat

You have two of those and want to add the doubles together?

Avremel Kaminetzky

@avremel

Yes

Rob Norris

@tpolecat

Yeah OptionT or Nested if you're using cats.

You don't need a monad transformer there so Nested is a better choice.

@ val a = Future(Option(42)) 
a: Future[Option[Int]] = Future(Success(Some(42)))

@ (Nested(a), Nested(a)).mapN(_ + _).value 
res7: Future[Option[Int]] = Future(Success(Some(84)))

Actually you can use the bare semigroup in this case.

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