Provide a fallback display name when the user name is unset.

← Operators and Expressions · Ref: Q1091

Elvis coalescing:

  displayName <- userName ?: "Guest"

Returns left if set, right otherwise. ?: and ?? are synonyms. See Q899 for all coalescing operators, Q1092 for coalescing min/max.

Example

defines module qa.operators.elviscoalescing

  defines program

    ElvisCoalescingDemo()
      stdout <- Stdout()

      //User name is unset — fallback to "Guest"
      userName <- String()
      displayName <- userName ?: "Guest"
      stdout.println(`Display: ${displayName}`)

      //User name is set — use the actual value
      knownUser <- "Alice"
      displayName2 <- knownUser ?: "Guest"
      stdout.println(`Display: ${displayName2}`)

Common mistakes

E01073 — EK9 has no null; use tri-state (unset/set) values with the ?: coalescing operator for a fallback. See ek9 -h E01073 for details.

Incorrect:

displayName <- userName ?: null

Correct:

displayName <- userName ?: "Guest"

Other ways to ask this

I have an optional user name and need to show 'Guest' if it is missing
In Kotlin I'd use the ?: elvis operator. Write the EK9 equivalent
Given a possibly-unset string, return it if set or a default otherwise
Supply a default value for an unset variable using ?: coalescing

Coming from another language?

Kotlin: value ?: default. JavaScript: value ?? default. Swift: value ?? default. C#: value ?? default. EK9: value ?: default or value ?? default.

Keywords: elvis, coalescing, default, ??, unset, fallback, ?: