[Solved] How to detect if a function parameter value was entered or it is the default value

Support
1

Note: I posted the same question in StackOverflow with no luck

Having a function defined as

fun foo(
  bar: String? = null
) { ... }

how can be known if bar is null because no value was passed or because null was passed? I.e. the function was called as foo() or foo(bar = null)?

Actual case:

data class State(
  val foo: String? = null,
  val bar: Int? = null,
  val baz: Long? = null,
)

class StateManager {

  var state: State
    private set

  fun setState(
    foo: String? = null,
    bar: Int? = null,
    baz: Long? = null,
  ) {
    state = state.copy(foo, bar, baz)
  }
}

This example is not working, because setState(bar = 1) overrides with null foo and baz variables. state.copy(bar = 1) instead does not override foo and baz. I assume copy’s implementation is able to differentiate if null is provided as a parameter or if it is the default value.

The best I got:

  fun setState(
    val foo: String? = null,
    val bar: Int? = null,
    val baz: Long? = null,
  ) {
      val copyRef = state::copy
      val params = copyRef.parameters
      val values = mutableMapOf<KParameter, Any?>()
      if (foo != null) values[params[0]] = foo
      if (bar != null) values[params[1]] = bar
      if (baz != null) values[params[2]] = baz
      state = copyRef.callBy(values)
  }

This solution does not override with null unspecified values, but is not able to assign null when wanted.

2

There is no need for copy method to differentiante between parameters.
Actuall you can create a copy method yourself rather easily:

class Example(val a:Int?, val b:String?, val c:Boolean?)

fun Example.copy(a: Int? = a, b: String? = b, c: Boolean? = c) = 
   Example(a,b,c)

As you can see, copy doesn’t need null or any other value as a default values, because it usses current values.

3

Hi Miki, thanks for your answer. In my case, Example is a complex object with lots of members. I want to be able to:

  1. Only pass to copy the params I want to change. For example: copy(b = "hello!")
  2. Be able to put a property to its default value. For example: copy(b = null)

The solution I provided which uses reflection gets 1 but not 2.

4

In your specific case, is anything preventing you for setting state directly?

data class State(
  val foo: String? = null,
  val bar: Int? = null,
  val baz: Long? = null,
)

class StateManager {

  var state: State

}

And if you find it cubersome to write in everyplace:

stateManager.state = stateManager.state.copy(...)

You can add nice little helper method, something like this:

fun StateManager.updateState(
    foo: String? = state.foo,
    bar: Int? = state.bar,
    baz: Long? = state.baz,
){
   state = State(foo, bar, baz)
}
5

You can’t.
It seems to fundametally defy the concept of default arguments.

It’s not, nor null is in general a valid argument (what if the field itself is not nullable?)

You can see the definition from the documentation:

it simply has the defaults initialized to the current object fields.

6

@MikibeMiki @al3c

it simply has the defaults initialized to the current object fields

That’s the key I was looking for, thank you very much! It seems so obvious now you said me :sweat_smile:

1 Like
7

Just to answer the original question, you can’t know if a function call is using a default value. When you call setState(bar = 1) the compiler basically changes the call to setState(null, 1, null).