How to write the following Kotlin contract?

Question

The question is very simple: (using Kotlin 1.3.71)

I have the following data alike this one:

data class Location(val lat: Double, val lng: Double)

I want to achieve a type-safety with a call like so:

val loc = location {
    lat = 2.0
    lng = 2.0
}

To achieve so I built:


fun location(builder: LocationBuilder.() -> Unit): Location {
    val lb = LocationBuilder().apply(builder)
    return Location(lb.lat!!, lb.lng!!)
}

data class LocationBuilder(
    var lat: Double? = null,
    var lng: Double? = null
)

To avoid having !! operators I would like to write a contract that helps the compiler infere a smartcast that says that the attributes lat and lng are not null but I have not been able to do that successfully.

I've tried things without success and I believe it might be because I am not fully understanding the dynamics of contracts. Those are the style of:



fun LocationBuilder.buildSafely(dsl: LocationBuilder.()->Unit): LocationBuilder {
    contract {
        returnsNonNull() implies (this@buildSafely.lat != null && this@buildSafely.lng != null)
    }
    apply(dsl)
    if(lat == null || lng == null) throw IllegalArgumentException("Invalid args")

    return this
}

fun location(builder: LocationBuilder.()->Unit): Location {
    val configuredBuilder = LocationBuilder().buildSafely(builder)

    return Location(configuredBuilder.lat, configuredBuilder.lng)
    /* I would expect a smart cast but I am getting a compile error stating that lat and lng may still be null */
}

So the question is:

Can this be done with the current Kotlin version? If so, how?

Does this answer your question? [How to make field required in kotlin DSL builders](https://stackoverflow.com/questions/53651519/how-to-make-field-required-in-kotlin-dsl-builders) — IlyaMuravjov, Apr 04 '20 at 20:57
Not really. It gets really close, but it uses methods in the builder DSL I would like to use properties as I show on the sample code provided — Some random IT boy, Apr 04 '20 at 21:35
Unfortunately, _contracts are allowed only for functions_, so you can't specify them for properties, at least for now. — IlyaMuravjov, Apr 04 '20 at 21:58

score 4 · Accepted Answer · answered Apr 01 '20 at 22:28

4

This is currently not possible. A contract can't be based on the properties of the class in the contract, so when you check latitude or longitude in the contract, that's not allowed.

answered Apr 01 '20 at 22:28

Francesc

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score 0 · Answer 2 · answered Jul 22 '20 at 06:25

You could just replace the '{}' for '()' and use named arguments:

val loc = Location(
  lat = 2.0
  lng = 2.0
)

Also note that this is the constructor of the class, no builder needed :) This works for all calls in Kotlin.

See https://kotlinlang.org/docs/reference/functions.html#named-arguments

score -1 · Answer 3 · answered Apr 02 '20 at 00:50

You can do something like this:

import kotlin.properties.Delegates

fun location(builder: LocationBuilder.() -> Unit): Location {
    val lb = LocationBuilder().apply(builder)
    return Location(lb.lat, lb.lng)
}


class LocationBuilder {
    var lat by Delegates.notNull<Double>()
    var lng by Delegates.notNull<Double>()
}

data class Location(
    val lat: Double,
    val lng: Double
)

fun main() {
    val l = location {
        lat = 2.0
        lng = 8.0
    }

    println(l)
}

But you won't have compilation time exception for the nulls. It means that it doesn't force you to set both properties (lat and lng)

How to write the following Kotlin contract?

Can this be done with the current Kotlin version? If so, how?

3 Answers3