Kotlin has no ternary operator (conditional operator)

Kotlin and ternary operator (conditional operator)

With Google's official support for Kotlin as the Android development language, Kotlin has been a hot topic lately, but Kotlin doesn't have a ternary operator.

val max = a > b ? a : b // NG

Instead, Kotlin if is treated as an expression, so you can write:

val max = if (a > b) a else b

But isn't it inconvenient that Java has a ternary operator but Kotlin doesn't? : thinking_face: So this time, I'd like to force something like a ternary operator: muscle:

Define a ternary operator

You can't use Operator overloading because Kotlin doesn't have the operators ? And :.

However, you can use ʻinfix` to define your own operator (what looks like). https://kotlinlang.org/docs/reference/functions.html#infix-notation

This time, define the class and infix function as follows.

data class TernaryOperation<out T>(val condition: Boolean, val value: T)

infix fun <T> Boolean.`?`(other: T) = TernaryOperation<T>(this, other)
infix fun <T> TernaryOperation<T>.`：`(other: T) = if (this.condition) this.value else other

? And : cannot be used as function names, so they are enclosed in backticks. However, although I could use `?`, I couldn't use `:` for some reason, so I chose `:` (full-width). Also, in Kotlin, you can use Japanese for function names as in Java, but ? You cannot use oror:`.

It looks like this when using the function defined above.

val max = (a > b) `?` a `：` b

It feels like that: sweat_smile:

Difference from Java ternary operator

Operator priority

Kotlin's infix function has a higher priority than the comparison operator. https://kotlinlang.org/docs/reference/grammar.html#precedence

On the other hand, Java ternary operators have a lower priority than comparison operators. https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html

Therefore, if you want to use a comparison operator, etc., like the ternary operator created this time, you need to enclose the conditional expression in parentheses.

Java

int max = a > b ? a : b; // OK

Kotlin

val max = a > b `?` a `：` b // NG

val max = (a > b) `?` a `：` b // OK
val max = (a > b) `?` a + 3 `：` b - 5 // OK

Evaluation of the formula

I noticed in the comment of saka1029. Thank you: bow:

Since the ternary operator mock created this time passes a value, both values will be evaluated regardless of the truth of the condition.

Java

int value = array.length > 0 ? array[0] : -1; // OK

Kotlin

val value = array.isNotEmpty() `?` array[0] `：` -1 //ArrayIndexOutOfBoundsException when array is empty

To avoid this, you have to make it ()-> T type instead of T type, but it is no longer different from if expression: sweat_smile:

data class TernaryOperation<out T>(val condition: Boolean, val value: () -> T)

infix fun <T> Boolean.`?`(other: () -> T) = TernaryOperation<T>(this, other)
infix fun <T> TernaryOperation<T>.`：`(other: () -> T) = if (this.condition) this.value() else other()

val value = array.isNotEmpty() `?` { array[0] } `：` { -1 } // OK

Patterns that use the Elvis operator

Define the following infix function.

infix fun <T> Boolean.then(other: T) = if (this) other else null

Using the Elvis operator, you can write it like a ternary operator as follows.

val max = (a > b) then a ?: b

However, if you use null for the value when true, it will always be the value when it is false: cry:

val body = response.isError() then null ?: response.body // always response.body

Also, as before, there are priority issues and expression evaluation issues.

Summary

I tried to make it with material, but it's still inconvenient: sweat: I also use Python a lot, so I'm still new to writing Kotlin ternary operators: sob:

If you have any impressions or opinions, please comment: bow: