Is there any Kotlin'ish way to use default values of function and constructor parameters programmati...

# announcements

[Marc Knaup]

Is there any Kotlin'ish way to use default values of function and constructor parameters programmatically or is this only possible with reflection and relying on the internals of the synthetic overloads (which also only works on the JVM)?

class Test(val a: Int = 1, val b: Int = 2, val c: Int = 3)

fun test() {
	var a: Int? = null
	var b: Int? = null
	var c: Int? = null
	if (someCondition) a = somethingNonNull
	if (someCondition) b = somethingNonNull
	if (someCondition) c = somethingNonNull

	Test(
		a = a ?: howToUseDefaultValue, // ??
		b = b ?: howToUseDefaultValue, // ??
		c = c ?: howToUseDefaultValue // ??
	)
}

[natpryce]

You can define the defaults as constants (vals) and refer to the constants within the body of the function.

[Marc Knaup]

No, I can't change the code of the file as I'm in annotation processing world 🙂
Also way too much unnecessary work.

[natpryce]

Ok. I thought you were writing the function itself

[Marc Knaup]

In some cases I am, but I'm looking for a general solution close to Kotlin world without reinventing defaults

[arekolek]

This is (kind of) what Moshi does:

fun test() {
    // ...

    val defaults = Test()
    Test(
        a = a ?: defaults.a,
        b = b ?: defaults.b,
        c = c ?: defaults.c
    )
}

[Marc Knaup]

only works if everything is optional which it isn't in most cases 😕

[Marc Knaup]

also can have nasty side-effects when the class
a) has a non-trivial initialzier
b) does something completely different in the default constructor

[Marc Knaup]

It's merely a workaround to the actual issue.
I'd prefer reflection in that case 😅

[arekolek]

Here’s the case when there’s required d

var result = Test(
        d = d ?: throw ...
)
result = Test(
        a = a ?: result.a,
        b = b ?: result.b,
        c = c ?: result.c,
        d = d ?: result.d)
return result

[Marc Knaup]

That's a way to create the instance indeed, but I'd make assumptions that there are no side-effects in the constructor / initializer 😕

[Marc Knaup]

Also found this:
https://youtrack.jetbrains.com/issue/KT-18695

[Marc Knaup]

Using KFunction.callBy now. Works great 🙂

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.reflect.jvm/reflect.html

[Marc Knaup]

https://github.com/fluidsonic/fluid-json/blob/master/annotation-processor/tests/cases/1/output-expected/property/defaultValue/AutomaticJSONCodec.kt

Looks horrible though 😅