Hey guys :slightly_smiling_face: Was looking to do...

# getting-started

[Jérémy CROS]

Hey guys 🙂
Was looking to do something like that but it doesn’t seem to be allowed

data class Foo(val a:Int, val b = 0)
val bar = Foo(a = 3, if(param) b = 5 /*else keep the default param */)

Is there an alternative other than val bar = if(param) Foo(...) else Foo(...)

[Holden Easley]

Its not pretty but something like

Foo(a = 3).let { 
  if (param) it.copy(b = 5) else it
}

??

[Jérémy CROS]

hmm, not sure I like it but it’s still an interesting alternative, thanks 🙂 will try with the actual code see how it feels

[Connor Ford]

maybe this is more readable?

data class Foo(val a: Int, val b: Int = DEFAULT_B) {
    constructor(a: Int, p: Boolean): this(a, if (p) a else DEFAULT_B)

    companion object {
        private const val DEFAULT_B = 0
    }
}

[CLOVIS]

Out of curiosity, what is the use-case? That seems like it would make the code much harder to understand

[ephemient]

https://youtrack.jetbrains.com/issue/KT-18695

[Stephan Schroeder]

how about

data class Foo(val a:Int, val b = 0)
val bar = if(param) {
  Foo(3, 5)
} else {
  Foo(3)
}

[ephemient]

with(Foo::class.primaryConstructor!!) {
    callBy(listOfNotNull("a" to 3, if (param) "b" to 5 else null).associate { (name, value) -> parameters.single { it.name == name } to value })
}

blob thinking upside down

[ephemient]

IIRC Moshi does the equivalent of

Foo::class.java
    .getDeclaredConstructor(Int::class.java, Int::class.java, Int::class.java, kotlin.jvm.internal.DefaultConstructorMarker::class.java)
    .newInstance(3, if (param) 5 else 0 /* this value doesn't matter */, if (param) 0 else 2 /* bitmask */, null)

[ephemient]

yeah you can see the implementation in https://github.com/square/moshi/pull/896

[Alex]

@Jérémy CROS, what does param refer to?

[Jérémy CROS]

It’s just a simple boolean with a business meaning.
The solution first suggested is actually decently readable when used with the actual code.

return ActionInfrastructureUntilLevel(
    buildAction(instructionHumanized.humanInstruction.action),
    buildDirectionalInfrastructure(
        instructionHumanized.humanInstruction.infrastructure
    ),
    infrastructureUntilLevel.level,
    infrastructureUntilLevel.label
).let {
    if (isFirstInstructionAnInfrastructure)
        it.copy(template = R.string.sentence_template_infrastructure_until_level_first)
    else it
}

it’s basically a sentence builder with optional template parameter that are later used for translation.
In that case, the simple if else as suggested by @Stephan Schroeder looks worse, IMO
Won’t comment on @ephemient trolling me though 😛

[ephemient]

heh. sorry about trolling with callBy, I couldn't resist. although I think the synthetic constructor trick is actually useful until KT-18695 has a resolution, it's not worth it unless you're dealing with a situation where you may have a combinatorial explosion of default args

[Jérémy CROS]

haha, yes. And thanks for the link btw, will monitor this one 🙂