How to auto apply init block after applying a func...

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[smallufo]

How to auto apply init block after applying a functional block ?

Take this code for example :

class Config(var degree: Double = 315.0) {
  init {
    require(degree > 180 && degree < 360)
    { "degree should between 180 and 360" }
  }
}

And if we initialize Config directly , it will check the value as expected :

@Test
fun normalTest() {
  try {
    Config(90.0)
    fail()
  } catch (e: Exception) {
    assertTrue(true)
  }
}

But if there is a function accepting the Config as functional object :

private fun calculate(block: Config.() -> Unit = {}) {
  val cfg = Config().apply(block)
  println("do something... with degree = ${cfg.degree}")
}

@Test
fun testCalculate() {
  try {
    calculate {
      degree = 90.0
    }
    fail("It SHOULD fail")
  } catch (e: Exception) {
    assertTrue(true)
  }
}

It will skip the init check block .
How to solve this problem ? Thanks.


I could only think of this solution :

class Config(var degree: Double = 315.0) {
  init {
    check()
  }

  fun check() {
    require(degree > 180 && degree < 360)
    { "degree should between 180 and 360" }
  }
}

and …

private fun calculate(block: Config.() -> Unit = {}) {
  val cfg = Config().apply(block).also { it.check() }
  println("do something... with degree = ${cfg.degree}")
}

But I think it is so ugly … Is there any better way ? Thanks.

[Ayfri]

You could use a value class for the degree property, in which you put the require in its init function like you did

[Rob Elliot]

init is called when the object is constructed. When you call Config().apply(block), the first thing that happens is Config(), with your default value for degree of 315.0. So the init is called, and passes, because 315 is between 180 and 360.

You then set degree to 90 - but you have no guard on changing degree.

Do you really want Config to be mutable? If so, and you want to keep the invariant that degree > 180 && degree < 360, you need to add it to `degree`’s setter.

[smallufo]

to @Ayfri: value class may not be an option , as there may be more properties in the Config class.
to @Rob Elliot: Config needs to be mutable , so that client of the function may call calculate like this

calculate {
  degree = 90.0
}

[Rob Elliot]

What’s wrong with Config(degree = 90.0) and make degree a val?

[smallufo]

Because there may be nested config , And I want to achieve something like ktor 's config (ex : https://ktor.io/docs/http-client-engines.html#java )

such as :

calculate (obj1 , obj2 ) {
  degree = 90.0
  angle { 
    value = 60.0
  }
}

[Rob Elliot]

I think this meets your requirements:

class Config(
   degree: Double = 315.0
) {
   var degree: Double = degree
      set(value) {
         require(value > 180 && value < 360)
         field = value
      }

   init {
      this.degree = degree
   }
}

[smallufo]

to @Rob Elliot , wow , thanks , I think it works . but a little ... cumbersome ... Is there any proposal to enforce such init checking after applying a block ?

[Rob Elliot]

No. Init happens on construction. apply is an extension function you call after the object has been constructed. Basically you pass the constructed Config instance as an argument to apply, so it absolutely has to already have been constructed.

[Rob Elliot]

The only way you can run logic when you reassign a field is in a custom setter for that field.

[smallufo]

Thank you very much 🙌

[Rob Elliot]

Config().apply { degree = 90 } is exactly the same as:

val config = Config()
config.degree = 90

[Ayfri]

Maybe they can overwrite the apply function ?

[ephemient]

class ConfigBuilder {
    var degree: Double = 315.0
}
class Config(val degree: Double)
fun Config(block: ConfigBuilder.() -> Unit): Config {
    val builder = ConfigBuilder()
    builder.block()
    return Config(builder.degrees)
}

then you can

Config { degree = 90.0 }

and a check in Config.init will happen after the block runs

[smallufo]

Thanks @ephemient: this is another beautiful solution. and it seems widely used in kotlin related libraries (ex : ktor) .