What is the ternary operator in EK9 and how do its ':' and 'else' forms differ?

← Control Flow · Ref: Q1360

EK9 has no if-EXPRESSION: 'if' is always a statement. When you want a value chosen by a condition, use the TERNARY operator (or a switch expression). The ternary is: 'condition <- valueIfTrue <separator> valueIfFalse', where the separator is either ':' or the keyword 'else' — the two forms are IDENTICAL in meaning.

TERNARY WITH ':'

  passMark <- 90
  grade <- score >= passMark <- "A" : "B"

Reads: if 'score >= passMark' then "A" else "B". (Comparison literals must be named — a bare 'score >= 90' is rejected with E11064 — so bind the threshold first.)

TERNARY WITH 'else'

  grade <- score >= passMark <- "A" else "B"

Identical result; choose whichever reads better. 'else' is often clearer in longer expressions; ':' is more compact.

ONLY THE SELECTED ARM IS PRODUCED

The condition, and each arm, are ordinary expressions; only the chosen arm's value becomes the result.

TERNARY IS NOT COALESCING

The ternary chooses on a Boolean condition. It is distinct from the coalescing operators, which choose on set-state: '?:' (isSet coalescing, 'a ?: b' yields a if a is set else b). Use a ternary for a genuine Boolean test; use '?:' when you mean 'a, or b if a is unset'.

WHY NOT if-as-expression
'if' stays a statement so that all value production is explicit (ternary or switch), keeping branch side-effects and value-selection visually separate.

See Q61 for the if statement. See Q68 for the switch expression (the other value-producing conditional). See Q243 for coalescing operators. See Q898 for isSet vs ternary.

Example

defines module qa.controlflow.ternarycolonelse

  defines program

    TernaryColonElseDemo()
      stdout <- Stdout()
      score <- 85
      passMark <- 90

      // ternary with ':' — condition <- valueIfTrue : valueIfFalse
      grade <- score >= passMark <- "A" : "B"
      stdout.println(`grade (colon form): ${grade}`)

      // ternary with 'else' — identical meaning, different separator keyword
      band <- score >= passMark <- "high" else "normal"
      stdout.println(`band (else form): ${band}`)

Common mistakes

E02001 — EK9 has no if-expression. Use the ternary 'condition <- valueIfTrue : valueIfFalse' (or 'else' instead of ':'), or a switch expression. 'if' is a statement only. See ek9 -h E02001.

Incorrect:

grade <- if score >= 90 then "A" else "B"

Correct:

passMark <- 90
      grade <- score >= passMark <- "A" : "B"

Coming from another language?

Java: 'cond ? a : b' — EK9's ':' form is the direct analogue but the condition is separated by '<-' ('cond <- a : b'). Java has no 'else' keyword variant. Python: 'a if cond else b' mirrors EK9's 'else' form. Kotlin/Rust/Swift: 'if' is itself an expression; EK9 deliberately keeps 'if' a statement and offers the ternary (and switch expression) for value selection so branch effects and value selection stay separate.

Keywords: expression, flow, select, value, else, conditional, ternary, colon, control, if-expression, coalescing