What are the coalescing operators in EK9?

← Operators and Expressions · Ref: Q899

EK9 has coalescing COMPARISON operators that return a value while handling UNSET gracefully. These are unique to EK9.

?? and ?: — VALUE COALESCING
Returns the left value if SET, otherwise the right:

  result <- maybeValue ?? defaultValue
  result <- maybeValue ?: fallback

COALESCING COMPARATORS (unique to EK9)
These COMPARE two values and RETURN a result. If either is UNSET, return the SET one:

  <? — coalescing minimum: returns the SMALLER value
  >? — coalescing maximum: returns the LARGER value
  <=? — coalescing less-or-equal minimum
  >=? — coalescing greater-or-equal maximum

Examples:

  bestPrice <- price1 <? price2    returns the cheaper price
  topScore <- score1 >? score2     returns the higher score

All coalescing operators RETURN a value — use them on the RIGHT side of <-.

NOTE: :=? is NOT a coalescing operator. It is an ASSIGNMENT operator (see Q1152). :=? assigns a value only if the target is unset. Do not confuse :=? (assignment) with <? (comparison).

See Q24 for tri-state semantics. See Q898 for the ? operator. See Q1226 for <? vs :=? contrast.

Example

defines module qa.operators.coalescingdetail

  defines function

    <?-
      Shows ?? value coalescing.
      Like Kotlin's ?: or JavaScript's ??
    -?>
    getDisplayName() as pure
      ->
        userName as String
        defaultName as String
      <-
        rtn as String: userName ?? defaultName

    <?-
      Shows <? coalescing minimum.
      Unique to EK9 — returns smaller, or the SET one.
    -?>
    cheaperPrice() as pure
      ->
        priceA as Float
        priceB as Float
      <-
        rtn as Float: priceA <? priceB

    <?-
      Shows >? coalescing maximum.
      Unique to EK9 — returns larger, or the SET one.
    -?>
    higherScore() as pure
      ->
        scoreA as Integer
        scoreB as Integer
      <-
        rtn as Integer: scoreA >? scoreB

  defines program

    CoalescingDemo()
      stdout <- Stdout()

      // ?? value coalescing — like Kotlin's ?: or JS's ??
      userName <- String()
      displayName <- getDisplayName(userName, "Guest")
      stdout.println(`Display: ${displayName}`)

      setName <- "Alice"
      displayName2 <- getDisplayName(setName, "Guest")
      stdout.println(`Display: ${displayName2}`)

      // <? coalescing minimum — RETURNS a value
      cheapest <- cheaperPrice(29.99, 19.99)
      stdout.println(`Cheapest: ${cheapest}`)

      // >? coalescing maximum
      topScore <- higherScore(85, 92)
      stdout.println(`Top: ${topScore}`)

Other ways to ask this

How do coalescing comparators work in EK9?
What do <? and >? mean in EK9?
What is the ?? operator in EK9?
How does null coalescing work in EK9?

Coming from another language?

Kotlin: ?: elvis operator — similar to EK9's ??. JavaScript: ?? nullish coalescing — similar to EK9's ??. Swift: ?? nil coalescing — similar to EK9's ??. C#: ?? null coalescing — similar to EK9's ??. None of these languages have coalescing COMPARATORS (<?, >?, <=?, >=?) — these are unique to EK9.

Keywords: null, comparator, coalescing, maximum, minimum, unset, elvis, coalesce