Creating your own if statement in Kotlin

By the nature of how lambdas are written in Kotlin, we can do write our own custom if statements.
Let’s start with a simple if-else statement, but in French, so it’s a si-sinon statement!

si-sinon Statement

We want it to look very similar to how if-else looks:

si (someCondition()) {
    println("true branch")
} sinon {
    println("false branch")
}

si and sinon are just functions and the code within braces ({ }) are just lambdas. The downside here is that we always need the braces to be used even for single line blocks. Here’s how the si and sinon functions can be defined:

class IfStatement(private val truth: Boolean) {
    infix fun sinon(onFalse: () -> Unit) {
        if (!truth) onFalse()
    }
}

fun si(truth: Boolean, onTrue: () -> Unit): IfStatement {
    if (truth) onTrue()
    
    return IfStatement(truth)
}

si-sinon Expression

Kotlin if-else statements can also be treated as an expression, wherein you can use the result of an if-else expression like assign it to a variable. Interestingly, there is no ternary operator in Kotlin, and if-else expression is the alternative.
Unfortunately, we need to keep the braces :-(.

val result = if (someCondition()) { 5 } else { 10 }

The following code makes the above syntax possible:

sealed class IfStatement<out T> {
    class True<T>(val trueValue: T) : IfStatement<T>()
    object False : IfStatement<Nothing>()
}

infix fun <T> IfStatement<T>.sinon(onFalse: () -> T): T =
    when (this) {
        is IfStatement.True -> trueValue
        IfStatement.False -> onFalse()
    }

fun <T> si(truth: Boolean, onTrue: () -> T) =
    if (truth) IfStatement.True(onTrue()) else IfStatement.False

else if (sinon si)

Let’s go a bit further and add support for else-if statements that look like:

si (someCondition()) {
    println("true branch")
} sinon si {
    println("another branch")
} sinon {
    println("false branch")
}

But, unfortunately, the best I could come up with is:

val x = si (3 < 2) {
    println("true branch")
}.`sinon si` (4 > 3) {
    println("another branch")
} sinon {
    println("false branch")
}

using the following code:

sealed class IfStatement<out T> {
    class True<T>(val trueValue: T) : IfStatement<T>()
    object False : IfStatement<Nothing>()
}

infix fun <T> IfStatement<T>.sinon(onFalse: () -> T): T =
    when (this) {
        is IfStatement.True -> trueValue
        IfStatement.False -> onFalse()
    }
    
fun <T> IfStatement<T>.`sinon si`(truth: Boolean, onTrue: () -> T) =
    if (truth) IfStatement.True(onTrue()) else IfStatement.False

fun <T> si(truth: Boolean, onTrue: () -> T) =
    if (truth) IfStatement.True(onTrue()) else IfStatement.False