Unit-Insertion works differently in Scala 2 & Scala 3

[Arthurm1]

Scala 3 handles unit-insertion differently to Scala 2

Compiler version

3.0.0 vs 2.13.6 - both using Scastie

Minimized code

val f: Boolean => Any = g => {
  if (g)
    1
}
println(f(true))
println(f(false))

Output - using Scala 3...

()
()

plus warning...
A pure expression does nothing in statement position; you may be omitting necessary parentheses

Expectation - as in Scala 2...

1
()

plus no warning

Comments

Scala 3 does give the above warning about a pure expression and changing the code to the following works...

val f: Boolean => Any = g => {
  if (g)
    1
  else
    ()
  }
}
println(f(true))
println(f(false))

From @smarter ...
"it looks like unit-insertion is working differently in scala 2 and 3 (in scala 2 it only inserts Unit in the missing branch, in Scala 3 it inserts it in both branches)"

[abgruszecki]

IIRC, I think this was intentional - an if with only one branch is essentially treated as a side-effectful statement. @odersky can you comment?

[odersky]

Yes, that's intentional. I am not sure whether Scala 2's behavior was an accident, or whether there was some specific motivation behind it.

[som-snytt]

The behavior is specified:

A short form of the conditional expression eliminates the else-part. The conditional expression if (e1e_1e1​) e2e_2e2​ is evaluated as if it was if (e1e_1e1​) e2e_2e2​ else ().

Also the feature is not called "Unit insertion" (which sounds helpful) but "Value discard" (which sounds possibly dangerous).

[cayhorstmann]

Is the Scala3 behavior documented anywhere? In http://dotty.epfl.ch/docs/reference/syntax.html, Expr1 ::= ... [‘inline’] ‘if’ Expr ‘then’ Expr [[semi] ‘else’ Expr] ..., so an if without else is an expression, not a statement. It's not immediately intuitive that it should be an expression with value ()

[som-snytt]

I saw a tailrec method of the form def f: Unit = if (b) f and wondered does that even compile. It compiles for Unit result but not for Any. The Scala 2 rule, that if (b) expr means if (b) expr else (), is much easier to reason about, in that there is nothing to reason about.

import annotation._

class C() {
  //type R = Unit
  type R = Any
  @tailrec
  final def f(i: Int): R =
    if (i > 0) {
      println(s"$i ...")
      f(i-1)   // Cannot rewrite recursive call: it is not in tail position
    }
}

object Test extends App {
  val c = new C()
  println(c.f(5))
}