scala/scala

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Rob Norris

@tpolecat

In FP world we define a kind of addition/multiplication over functions and use that very general mechanism to combine them. It's a good observation you made and it leads to a lot of beautiful stuff.

RoelofWobben

@RoelofWobben

@tpolecat thanks :

def filterBoth(numberOfFilms : Int, year : Int):Seq[String] = bornBefore(year).filter(filterOnNumberFilms(numberOfFilms).contains)

yep, I know , Before I came here , I learned haskell and learned a lot about re-using functions or as they called it function composition if im right

Dominic Egger

@GrafBlutwurst

in FP scala a lot of the inspiration comes from haskell, i'm sure you'll see a lot of thigs you'll be familiar with

RoelofWobben

@RoelofWobben

yep, monads, functors , applicative , recursion, map, foldmap are a few

the first three are explained in the cats book

the syntax is different and that is the most difficult to pick up

@GrafBlutwurst

RoelofWobben

@RoelofWobben

time to sleep here , see you all tomorrow

Jason Pickens

@steinybot

I’m having some problems with path dependent types. Not sure if what I am trying to do makes sense. Anyway here is an example:

sealed trait Foo {
  type F
}

object Bar extends Foo {
  type F = Array[Byte]
}

trait Baz[B] {
  def b(b: B): String
}

object Baz {
  implicit val bazByteArray: Baz[Array[Byte]] = (b: Array[Byte]) => new String(b)
}

def f(a: Foo): Baz[a.F] = {
  val baz = a match {
    case bar@Bar => g(bar)
  }
  baz
} // Expression of type Baz[(a.type with Bar.type)#F] doesn't conform to Baz[a.F]

def g(a: Foo)(implicit baz: Baz[a.F]): Baz[a.F] = {
  baz
}

val x1: Bar.type = Bar
val y1: Baz[x1.F] = f(x1)

val x2: Foo = Bar
val y2: Baz[x2.F] = f(x2) // Expression of type Baz[Foo#F] doesn't conform to expected type Baz[x2.F]

What I don’t get is why g compiles but f doesn’t. Are the types not the same?

zetashift

@zetashift

Just a question out of curiosity, making a function a first class value you need to add an apply method(function?), why is it named apply?

Mathieu Prevel

@mprevel

object Test {
  def apply() = doSomething()
}

can be called Test()

this is a special case

Ichoran

@Ichoran

Ugh, this expression stuff is so clunky in this FP example.

There are perfectly reasonable OO ways to make the whole thing tidy, e.g.

trait Expr { def eval: Either[String, Double] }
sealed trait BinOp extends Expr {
  def a: Expr
  def b: Expr
  def op: (Double, Double) => Either[String, Double]

  protected def safe(f: (Double, Double) => Double): (Double, Double) => Either[String, Double] = 
    (xa, xb) => Right(f(xa, xb))

  def eval = a match {
    case Left(s) => Left(s)
    case Right(xa) => b match {
      case Left(s) => Left(s)
      case Right(xb) => op(xa, xb)
  }
}
final case class Add(a: Expr, b: Expr) extends BinOp { val op = safe(_ + _) }
final case class Sub(a: Expr, b: Expr) extends BinOp { val op = safe(_ - _) }
final case class Mul(a: Expr, b: Expr) extends BinOp { val op = safe(_ * _) }
final case class Div(a: Expr, b: Expr) extends BinOp {
  val op = (xa, xb) => if (xb == 0) Left("Division by 0") else Right(xa / xb)
}

Seth Tisue

@SethTisue

@zetashift because that’s what you’re doing, applying the function. it isn’t the only possible word that could have been chosen, for example JavaScript has apply but it also has call, Lisp has funcall, and so forth. Java also calls it apply

Fabio Labella

@SystemFw

the two solutions aren't equivalent with respect to extensibility @Ichoran . Expression problem and that

ime the OO one is the side of it you want less often

(final tagless can solve both sides, but that's a separate discussion)

Ichoran

@Ichoran

I don't know, I've written half a dozen expression evaluators and never once wanted to make the expressions extensible outside of the code that defines the operations in a way that isn't trivially handled by inheritance.

Fabio Labella

@SystemFw

you never wish to add a pretty printer and an evaluator?

adding ops is fairly common

adding nodes, less so

Ichoran

@Ichoran

Not outside of the evaluator, no.

I can of course envision cases where it's helpful, but I haven't run into them.

Normally, I do it so that end-users won't have to bother thinking about what makes for a correct expression or how to pretty-print things; that's the point of me writing it at all.

Hopefully the FP example eventually gets to the point where it is helpful instead of just awkward?

Fabio Labella

@SystemFw

Normally, I do it so that end-users won't have to bother thinking about what makes for a correct expression or how to pretty-print things; that's the point of me writing it at all.

I don't understand this

as in , can't parse it

Ichoran

@Ichoran

So, I generally write expression trees like this when I want to offer a DSL as part of the API. The reason I do this is so I can think through the nonobvious stuff like what operations are valid and handle all the corner cases for the consumer of the DSL.

Abdhesh Kumar

@abdheshkumar

Just use flatMap on Future[Option[Double]]

Ichoran

@Ichoran

Extensibility implies that they get access to all the corners upon which they can poke themselves.

Avremel Kaminetzky

@avremel

@abdheshkumar thanks,

Ichoran

@Ichoran

Does that make more sense, @SystemFw ?

Fabio Labella

@SystemFw

well, if you're making a DSL, being able to interpret it seems reasonable

but the thing is

if you never need to extend it in any direction

the two approaches are exactly equivalent

except with the OO one the code logic is more scattered

rather than in one place like pattern matching

Abdhesh Kumar

@abdheshkumar

val r: Future[Option[Double]] = ???
r.flatMap{
case Some(v)=> Future.successful(v)
case None=> Future.failed(new Exception)}

Ichoran

@Ichoran

The pattern matching has a ton of repetition.

Fabio Labella

@SystemFw

where?

Ichoran

@Ichoran

Well, I guess I haven't seen what the gold-standard answer is supposed to be.

Fabio Labella

@SystemFw

I think the repetition there is mostly due to not being able to work with Either idiomatically because the appropriate combinators haven't been introduced yet

so a lot of matching on Left and Right

Ichoran

@Ichoran

Even then it's more machinery to get back to the same cleanliness of specifying operations.

Fabio Labella

@SystemFw

transforming things through pattern matching (complex trees) can indeed be tedious (that's where recursion schemes come into play), but I think that when you reach that point, the OO approach is hopeless anyway

@Ichoran I guess it depends on what you mean by cleanliness. For me being able to see all the logic for a given interpreter in one place is cleaner than it being scattered

Ichoran

@Ichoran

Yeah, OO is not well-suited to expression-simplification because it has very local perspective.

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