Hi all, I have a question about extension function...

# announcements

[Juan B]

Hi all, I have a question about extension functions, I'm kind of stuck. Hope anyone can point me in the right direction.

The problem I'm trying to solve: I want to be able to check when a property in a data class has been set. The goal is to be able to differentiate between a null value (the property has been set to null) and a non present (value was not set). A potential use case is to implement a HTTP Patch where you receive Input Objects as DTO and you want to save in the database entities only the properties set in the DTO.

My initial approach: I thought it could be solve with a delegate using a map, so I could use the map to check if the value was actually set. Here is some code, but it is not working.

class InputExample(val map: Map<String, Any?>) {
        val name: String? by map
        val age: Int?     by map
        val lastname: String? by map

        fun KProperty<Any>.isPresent(): Boolean {
            //return map.containsKey(this.name)
            return true
        }
 }

I was trying to get to a interface like this:

inputType.lastname.isPresent()

[ephemient]

::lastName.isPresent() could work, but ... did you mean to write val name: String? by map or val name: String by map.withDefault { "?" } instead of something that will throw up on non-present value?

[Juan B]

Yes, sorry. It will be something like

val name: String? by map

(I just updated the code)

I tried to use it like this:

assert(!inputType::lastname.isPresent())

but I got a "Unresolved reference: isPresent"

[Juan B]

BTW, any other implementation idea is welcome. I'm pretty new to Kotlin 😅

[Nir]

not familiar with reflection based approaches

[Nir]

But offhand seems like the way to go is basically an optional like class

[Nir]

A generic class that stores a T? and a bool

[Nir]

Because nulls coalesce and T?? is the same type as T? sometimes you have to live with this in Kotlin

[Juan B]

I'm not sure if I'm following the solution.I'm sorry. I may be missing some fundamentals.

[Juan B]

are you referring to Java Optionals?

[Roukanken]

the problem why it doesn't work is
that in a function like defined like this, there have to be TWO this - one of type InputExample, and one of type KProperty<Any>

so if you wanted to call your function, you would have to do smth like this:

val test = InputExample(mapOf(
    "name" to null,
    "age" to 4,
    "lastname" to "Lastname",
))

with(test) {
    ::name.isPresent()   // true
}

(after you edit the isPresent to be callable on KProperty<Any?>, because KProperty<String?> is not subtype of non nullable KProperty<Any>)

[Roukanken]

if you did just

test::name.isPresent()

then this is only one - test::name which is KProperty<String?>
but the InputExample this is missing, that's why you can't call the function

[Timmy]

The java.util.Optional approach (val name : Optional<String?>) doesn't work because Optional cannot contain "null". So your are left with Optional<String>? which seems rather weird.

Instead we can create our own Option class that does support this use case:

sealed interface Maybe<out T>
object Missing : Maybe<Nothing> {
    override fun toString(): String = "Missing"
}
data class Present<T>(val value : T) : Maybe<T>

Arrow kt probably has an implementation of this somewhere.

So now the example is:

data class Example(
    val name : Maybe<String?>,
    val age : Maybe<Int?>,
    val lastName : Maybe<String?>
)

val foo = Example(Missing, Present(null), Present("bar"))

and

inputType.lastname is Present

With a few helper methods (Maybe<T>.get() : T, Maybe<T>.getOrNull() : T?, etc.) it becomes manageable.

This is the solution I picked for the scenario of PATCH requests where missing and null are different.
I also have a module for Jackson to get the JSON correct (i.e. absent is not written, null is written as null). If you want I can share that with you, but it is quite hairy

[Youssef Shoaib [MOD]]

Also also, as an alternative approach, you can make this much less complicated by simply having a custom setter for each field that sets a boolean value like this:

class InputExample(name: String?, age: Int?, lastname: String?) {
    val nameIsPresent: Boolean = false
    val name: String? get() = field 
        set(value) { 
            nameIsPresent = true
            field = value
        }
    ...
}

[Nir]

@Timmy I'm not suggesting java.util.Optional

[Nir]

you'd write your own optional

[Nir]

class Optional<T>(val isValid: Boolean, val t: T?)

basically

[Nir]

yeah, like what you wrote after

[Nir]

okay, I missed Juan mentioning Java's optional, I understand now 🙂

[Nir]

I don't agree that Youssef's approach is much less complicated, you'll be repeating the exact same code for each field, makes more sense to factor this into a small class

[Juan B]

Thanks for the help. I'm going to explore the options you've shared.

@Timmy I would love to see the library you mentioned if you don't mind. Thanks for the offer!

[Timmy]

Here you go. It is part of a bigger library, which I can't share, but that is the relevant part

[Juan B]

Thanks!