Hi guys, I have some trouble with Kotlin, more the...

# exposed

[Gustav Elmgren]

Hi guys, I have some trouble with Kotlin, more then exposed maybe, so it might be the wrong channel.


But lets say I have the following:

Flights
    .join(...)
    .join(...)

And then I have an optional join based on another parameter. One solution I guess is to use a var:

var joins = Flights
    .join(...)
    .join(...)

if (condition) { joins = joins.join(AnotherTable...) }

Is there any feature in kotlin that make it possible to do the if in the assignment so I can keep it as a val ? (or something like adjustWhere in Exposed)

Flights
    .join(...)
    .join(...)
    .someMagicKotlinMethod {
       if (condition) ...
    }

[phldavies]

.let { if (condition) it.join(other) else it } (https://kotlinlang.org/docs/scope-functions.html#let)

[Kamil Kalisz]

Hey @Gustav Elmgren

you can create some extension method.

fun Join.conditional(condition: Boolean, conditionalBlock: (Join) -> Join): Join{
    return if(condition) {
        conditionalBlock.invoke(this)
    } else this
}

and use it in this way:

NoteTable.join(EarthTable, joinType = JoinType.INNER)
    .conditional(true){
        it.crossJoin(WaterTable)
    }.conditional(false){
        it.fullJoin(SunTable)
    }.join(IceTable, joinType = JoinType.CROSS)

[Kamil Kalisz]

or version with lambda receivers

fun Join.conditional(condition: Boolean, conditionalBlock: Join.() -> Join): Join{
    return if(condition) {
        conditionalBlock.invoke(this)
    } else this
}

NoteTable.join(NoteTable, joinType = JoinType.INNER)
    .conditional(true){
        crossJoin(NoteTable)
    }.conditional(false){
        fullJoin(NoteTable)
    }.join(NoteTable, joinType = JoinType.CROSS)

[Gustav Elmgren]

Awesome, thank you!

[phldavies]

a slightly more generic form would be

inline fun <T> T.letIf(condition: Boolean, block: (T) -> T) = if(condition) let(block) else this

or

inline fun <T> T.runIf(condition: Boolean, block: T.() -> T) = if(condition) block() else this

(named consistently with `let`/`run` )

example usage:

Flights
    .join(...)
    .runIf(condition) { join(...) }
    .letIf(condition) { it.join(...) }

[Kamil Kalisz]

Just remeber about returning new value

[phldavies]

They do return the new value (fixed a typo in the letIf) - an more (contrived) example: https://pl.kotl.in/iLYHl-LOH