`fun returnsUnit() = if (condition) { doSomething }` …does not compile, but `fun returnsUnit() = if ...

# getting-started

[y]

fun returnsUnit() = if (condition) { doSomething }
…does not compile, but
fun returnsUnit() = if (condition) { doSomething } else { }
does. I understand why it is like this, but is this considered a bug?

[Adam S]

https://kotlinlang.org/docs/control-flow.html#if-expression

If you’re using if as an expression, for example, for returning its value or assigning it to a variable, the else branch is mandatory.

[y]

I understand why it is like this, but some languages (Rust is one I know of) do allow this for Unit return values (by implicitly adding a else { return Unit })

[Adam S]

this is the closest feature request I could find https://youtrack.jetbrains.com/issue/KT-18694

[Adam S]

understood - but it’s intended behaviour, not a bug

[y]

okay. thanks for digging up that issue. seems like it’s either intended behavior or at the very least not a priority

[Adam S]

the empty else {} isn’t necessary if the function has a block body

fun returnsUnit() {
  if (condition) {
    doSomething
  }
}

[ephemient]

it isn't necessary in a Unit-returning lambda either

[Stephan Schröder]

I always assumed that an if branch without an else branch is simply a statement and not an expression!? The reason it works at the end of a lambda or function returning Unit is that both get a return Unit implicitly added to it anyway.
At least that is my mental model which seems to comply with actual behaviour (compiler implementation may differ).

[ephemient]

right. all expressions can be statements, but not all statements are expressions. declarations, loops, and non-exhaustive when are also not expressions

[y]

now that I understand the degree to which Kotlin is “expression-oriented” I guess the if-without-else here is not necessarily an inconsistency. not when something like non-exhaustive when is allowed by the compiler.

[ephemient]

to a degree, Rust started life as OCaml with C-like syntax, while Kotlin started life as Java improved. so while modern language design has converged in some ways, the expression/statement divide is one of those that is still a bit different.