Does EK9 have ++ and -- operators? What are the rules?

← Operators and Expressions · Ref: Q967

YES, EK9 has ++ and -- operators. But they are STATEMENT-ONLY — they cannot be used in expressions.

VALID (statement-only):

  count++         increments count by 1
  count--         decrements count by 1
  temperature++   increments temperature

INVALID (not an expression):

  y <- x++        COMPILE ERROR — ++ cannot be used in an expression
  total <- count++ + 5    COMPILE ERROR
  if x++ > 10     COMPILE ERROR

WHY STATEMENT-ONLY?

The mutating operators (++, --, +=, -=) return no value - they change the existing object in place and are void. So there is nothing for an expression to use, and writing one is E07950. Because they mutate in place, remember that a plain y <- x shares ONE object between y and x: a later x++ or x += 5 changes what y shows too. When you need an independent value, take a copy: y <- Integer(x).

Bridge: Like C++/Java's ++ but restricted to statements. Similar to how Python chose not to have ++ at all — EK9 allows it, but only as a statement that returns no value.

OTHER MUTATING OPERATORS:

  x += 5    statement-only, adds 5 to x
  x -= 3    statement-only, subtracts 3 from x
  x *= 2    statement-only, multiplies x by 2

All mutating operators follow the same rule: statement-only, no expression use.

See Q905 for mutating operator aliasing details. See Q241 for the full mutation operator list.

Example

defines module qa.operators.incrementrules

  defines program

    IncrementDemo()
      stdout <- Stdout()

      // ++ as statement — VALID
      counter <- 0
      counter++
      stdout.println(`After ++: ${counter}`)

      counter++
      counter++
      stdout.println(`After two more ++: ${counter}`)

      // -- as statement — VALID
      counter--
      stdout.println(`After --: ${counter}`)

      // += as statement — VALID
      counter += 10
      stdout.println(`After += 10: ${counter}`)

      // -= as statement — VALID
      counter -= 3
      stdout.println(`After -= 3: ${counter}`)

      // The correct pattern when you need the value:
      // Increment first, then use the variable
      score <- 100
      score++
      displayScore <- score
      stdout.println(`Score after increment: ${displayScore}`)

      // NOT: displayScore <- score++  (COMPILE ERROR)

Common mistakes

E07950 — The '++' operator is statement-only in EK9 — it cannot be used inside an expression like 'displayScore <- score++'; increment on its own line first, then use the variable. See ek9 -h E07950 for details.

Incorrect:

      displayScore <- score++

Correct:

      displayScore <- score

Coming from another language?

Java: i++ works in expressions (causes bugs). C: i++ is expression (undefined behaviour in some cases). Python: no ++ at all. EK9: ++ exists but is statement-only — the safest of all approaches.

Keywords: statement only, mutation, increment, decrement, aliasing