Using Iron for newtypes to get a compile error when using A in place of B

[Anoia]

I'd like to use Iron for newtypes, for now just to prevent usages of A in place of B, when A and B have the same type but different meanings in my domain.

I tried one of the examples from the docs, but I don't get a compile error where I expected one:

import io.github.iltotore.iron.*
import io.github.iltotore.iron.constraint.numeric.Positive

opaque type Temperature = Double :| Positive
object Temperature extends RefinedTypeOps[Double, Positive, Temperature]

opaque type Moisture = Double :| Positive
object Moisture extends RefinedTypeOps[Double, Positive, Moisture]

case class Info(temperature: Temperature, moisture: Moisture)

@main
def main(): Unit =
  val temperature: Temperature = Temperature(20.0)
  val moisture: Moisture = Moisture(0.5)

  val info = Info(moisture, temperature) // should not compile but does?

  println(info)

I use iron version 2.6.0 and Scala 3.4.2. I compiled it via sbt. The example is from the iron docs.

this compiles as well for me, it's from the other example in the docs

import io.github.iltotore.iron.*
import io.github.iltotore.iron.constraint.all.Positive

object Test:
  opaque type Temperature = Double :| Positive
  object Temperature extends RefinedTypeOps[Double, Positive, Temperature]
  val x: Double :| Positive = 5
  val temperature: Temperature = x // compiles as well

is there something I'm missing?

[Iltotore]

Hello. This is due to how opaque types work. They are only opaque outside of their definition scope (like in a different file) and that's why the example is split in two code samples (tho it might be useful to add a reminder about this scope mechanism).

Here is a working version in Scastie. Since it (Scastie) does not support multiple files, I wrapped the types in an object:

import io.github.iltotore.iron.*
import io.github.iltotore.iron.constraint.numeric.Positive

object types:
  opaque type Temperature = Double :| Positive
  object Temperature extends RefinedTypeOps[Double, Positive, Temperature]

  opaque type Moisture = Double :| Positive
  object Moisture extends RefinedTypeOps[Double, Positive, Moisture]

import types.*

case class Info(temperature: Temperature, moisture: Moisture)

@main
def main(): Unit =
  val temperature: Temperature = Temperature(20.0)
  val moisture: Moisture = Moisture(0.5)

  val info = Info(moisture, temperature) // should not compile but does?
  val info2 = Info(temperature, moisture) // OK

  println(info)

[Anoia]

Thanks, that makes so much sense! 🤦🏼