Does ++ return a value in EK9? Can I write x <- y++?

← Operators and Expressions · Ref: Q1057

In EK9, ++ and += are mutation operators. They modify the variable in place and return NOTHING. You cannot use them in expressions.

++ INCREMENTS IN PLACE:

  count <- 0
  count++          modifies count to 1, returns nothing

+= ADDS IN PLACE:

  score <- 10
  score += 5       modifies score to 15, returns nothing

WHAT YOU CANNOT DO:

  x <- count++     WRONG — ++ returns nothing, cannot assign
  total <- a += b  WRONG — += returns nothing, cannot assign
  if count++ > 5   WRONG — ++ returns nothing, cannot compare

WHAT TO DO INSTEAD:

  count++
  x <- count       increment then read separately

All mutation operators return void: ++, --, +=, -=, *=, /=, :=:, :^:, :~:

Mutation operators CANNOT be marked 'as pure' because they change state. A pure function must not mutate.

See Q241 for all mutation operators. See Q908 for $ operator. See Q1055 for default vs manual operators.

Example

defines module qa.operators.incrementmutator

  defines program

    IncrementDemo()
      stdout <- Stdout()

      count <- 0
      stdout.println(`Before: ${count}`)

      //++ mutates in place, returns nothing
      count++
      stdout.println(`After ++: ${count}`)

      //+= mutates in place, returns nothing
      count += 10
      stdout.println(`After += 10: ${count}`)

      //-= also mutates in place
      count -= 3
      stdout.println(`After -= 3: ${count}`)

Common mistakes

E07950 — ++ mutates in place and returns nothing, so it cannot appear inside an expression such as string interpolation; increment first, then read the variable. See ek9 -h E07950 for details.

Incorrect:

Before: ${count++}

Correct:

Before: ${count}

Coming from another language?

Java/C/Go: ++ returns a value (pre/post increment). Python: no ++ operator. EK9: ++ mutates in place, returns nothing — cannot be used in expressions.

Keywords: plus, operator, expression, mutator, void, increment