Is the ? operator prefix or suffix in EK9?

← Operators and Expressions · Ref: Q936

The ? operator is always SUFFIX in EK9. Write name? not ?name.

SUFFIX SYNTAX

  if userName?
    stdout.println(userName)

The ? comes after the variable, like Rust's .is_some() or Swift's optional unwrapping.

WHY SUFFIX

Suffix reads naturally as a question: 'is userName set?' The ? asks a question about the thing to its left. Prefix ? would be ambiguous with other operators.

USAGE CONTEXTS

- `variable?` asks: is this variable set?
- `list?` asks: is this list set? (always true for created lists)
- `result?` asks: is this result set?
- `if connection?` is a guard: only enter if connection is set

COMBINING WITH NOT

  if not userName?
    stdout.println("No user name")

The pattern 'not variable?' reads as 'not (variable is set)'.

See Q877 for isSet details. See Q898 for more ? examples. See Q29 for tri-state. See Q2253 for why '+=' or '++' can never give an unset value a value (E08096).

Example

#!ek9
defines module qa.operators.issetsuffix

  defines program

    IsSetSuffixDemo()
      stdout <- Stdout()

      //Unset variable — ? returns false
      greeting <- String()
      stdout.println(`Greeting is set: ${greeting?}`)

      //Give it a value (known to be unset here, so ':='), then the suffix check again
      greeting := "Hello"
      stdout.println(`Greeting is set: ${greeting?} - ${greeting}`)

      //Works on any type
      score <- Integer()
      stdout.println(`Score is set: ${score?}`)

      score := 42
      stdout.println(`Score is set: ${score?} - ${score}`)

Coming from another language?

EK9 uses value? as a suffix — append ? after the variable name to check if it is set. Returns Boolean, no parentheses needed.

Keywords: check, isset, mark, operator, position, question, set, suffix