‘if … else if’ expression unexpectedly has Unit value when last branch taken

Compiler version

3.1.1

Minimized code

This code prints (), as a evaluates to the Unit value:

 val a =
    if false then
       1
    else if true then
       2
 
 println(a)

Update: this code may be a bit too minimized to demonstrate the scope of the actual problem. See the examples in this comment for more clarity.

Output

()

Expectation

Although this code is unusual and unlikely to be written by an experienced programmer (it is a simplified and minimized version of something written by my 8-year-old son who has just started learning to program with Scala), I suspect most people would intuitively expect a to evaluate to 2 rather than to ().

Adding an unconditional else clause to the if statement restores expected behaviour:

val a =
    if false then
       1
    else if true then
       2
    else
       3
 
 println(a) // prints 2

[som-snytt]

@KacperFKorban hi, if you agree, would you mind reopening? My previous judgment may have been premature. Or, we could start a discussion in discussions.

[som-snytt]

That’s pretty funky. Just the penultimate branch (before the missing else) gets its value discarded (as I now see you said). That must be a glitch.

➜  dotty git:(test/current) ./bin/scala -Vprint:parser,typer
Welcome to Scala 3.1.3-RC1-bin-SNAPSHOT-git-f3cca47 (17.0.2, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> def f(x: String) = {
     |   if x == "1" then 1
     | }
[[syntax trees at end of                     typer]] // rs$line$1
package <empty> {
  final lazy module val rs$line$1: rs$line$1 = new rs$line$1()
  final module class rs$line$1() extends Object() { this: rs$line$1.type =>
    def f(x: String): Unit =
      {
        if x.==("1") then
          {
            1
            ()
          }
         else ()
      }
  }
}

1 warning found
def f(x: String): Unit
-- [E129] Potential Issue Warning: ------------------------------------------------------------------------------------------------------
2 |  if x == "1" then 1
  |                   ^
  |                   A pure expression does nothing in statement position; you may be omitting necessary parentheses
  |
  | longer explanation available when compiling with `-explain`

scala> def f(x: String) = {
     |   if x == "1" then 1
     |   else if x == "2" then 2
     |   else if x == "3" then 3
     | }
[[syntax trees at end of                     typer]] // rs$line$2
package <empty> {
  final lazy module val rs$line$2: rs$line$2 = new rs$line$2()
  final module class rs$line$2() extends Object() { this: rs$line$2.type =>
    def f(x: String): AnyVal =
      {
        if x.==("1") then 1 else
          if x.==("2") then 2 else
            if x.==("3") then
              {
                3
                ()
              }
             else ()
      }
  }
}

1 warning found
def f(x: String): AnyVal
-- [E129] Potential Issue Warning: ------------------------------------------------------------------------------------------------------
4 |  else if x == "3" then 3
  |                        ^
  |                        A pure expression does nothing in statement position; you may be omitting necessary parentheses
  |
  | longer explanation available when compiling with `-explain`

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